Includes combined imports of assertion methods and related polishing.

Closes #22889

This commit deprecates MediaType.APPLICATION_JSON_UTF8 and
MediaType.APPLICATION_PROBLEM_JSON_UTF8 in favor of
MediaType.APPLICATION_JSON and MediaType.APPLICATION_PROBLEM_JSON since
UTF-8 encoding is now handled correctly by most browsers
(related bug has been fixed in Chrome since September 2017).

MediaType.APPLICATION_JSON is now used as the default JSON content type.

Closes gh-22788

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6515 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6625 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

MockServerWebExchange now has a second factory method that accepts a
MockServerHttpRequest.BaseBuilder.

Issue: SPR-16079

Discovered late, but not too late. MockServerWebExchange is now in the
proper package matching to the location of ServerWebExchange.

The revised builder emphasizes creating a list of resolvers either
built-in or custom with each top-level builder method resulting in
adding a resolver.

By default only the Header resolver is configured.

The path extension resolver is removed altogether to discourage its use
but is trivial to create manually with the helpf of
UriUtils#extractFileExtension + MediaTypeFactory.

Issue: SPR-15639

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

This commit folds ServerHttpMessage[Reader|Writer] into its parent
HttpMessage[Reader|Writer] with the server methods pre-implemented
by default to be simple pass-through delegates.

CharSequenceEncoder now supports all MIME types, however since encoding
Flux<String> can overlap with other encoders (e.g. SSE) there are now
two ways to create a CharSequenceEncoder -- with support for text/plain
only or with support for any MIME type.

In WebFlux configuration we insert one CharSequenceEncoder for
text/plain (as we have so far) and a second instance with support for
any MIME type at the very end.

Issue: SPR-15374

ServerHttpMessage[Reader|Writer] are now expected for WebFlux,
annotated controller endpoint and subsequently the instanceof checks
for HttpMessage[Reader|Writer] vs ServerHttpMessage[Reader|Writer] can
be removed from AbstractMessageReaderArgumentResolver and
AbtractMessageWriterResultHandler.

This commit updates the test code base to conform to changes in media
types returned by the MediaTypeFactory.

Issue: SPR-14908

Issue: SPR-15350

From spring-webflux to spring-web test sources since it is perfectly
usable for testing Spring MVC annotation method support.

Potentially to be promoted further up for use in any module that has
annotation method support. It has spring-core dependencies only

Before this change the write Publisher was saved and Mono.empty()
returned from the write metohd which did not properly implement
the write contract since no writing ("consuming") was done.

This can be a problem in some cases. For example the request may appear
to succeed even if the publisher produces an error later when
subscribed to later after request handling completes.

This commit introduces a writeHandler function in the mock request and
response. By default it "writes" by consuming the content immediately,
which allows it to return a Mono<Void> that properly reflects when
writing is done, and it also caches the data so it may be replayed
later for test assertions.

For streaming scenario a custom writeHandler may be registered which
allows the custom handling to determine how long to stream before
cancelling so request handling may complete.

Issue: SPR-14590

Issue: SPR-15190

MockServerHttpRequest and MockServerHttpResponse now extend the same
abstract base classes that server-specific implementations do and
therefore approximate their behavior more closely.

As an immediate consequence MockServerHttpRequest is read-only after
it is created. Instead it now exposes static builder methods similar
to those found in RequestEntity. This enforces more strictness as well
as recycling of requests in tests and provides nicer builder methods.

To simplify tests DefaultServerWebExchange now offers a constructor
with just a request and response, and automatically creating a
DefaultWebSessionManager.

The spring-test module now also contains client-side reactive mock
request and response implementations. The mock client request extends
the same AbstractClientHttpRequest as client-specific implementations
do. There is no abstract base class for client responses.

Issue: SPR-14590

If the body class is not resolvable from the return type and there is
a body instance we now fall back on the class of the body instance.

Issue: SPR-14877

Issue: SPR-14800

 - ScriptedSubscriber has been renamed to Verifier
 - The Publisher is passed to create() instead of verify()
 - No more need to specify the generic type explicitly
 - Version is now sync with reactor-core

Issue: SPR-14800

Reactor recently added the `ScriptedSubscriber` in its new
`reactor-addons` module. This `Subscriber` revissits the previous
`TestSubscriber` with many improvements, including:

* scripting each expectation
* builder API that guides you until the final verification step
* virtual time support

This commit refactor all existing tests to use this new
infrastructure and removed the `TestSubscriber` implementation.

Issue: SPR-14800

This change puts the MockHttpServerRequest/Response under
org.springframework.mock.http.server.reactive.test which
mirrors the arrangement we have with the Servlet mock equivalents
and sets up the addition of MockHttpServerRequest/Response in
spring-test.

Issue: SPR-14421

Issue: SPR-14421

This commit adds support for RxJava 2 Completable,
Single, Observable and Flowable types (io.reactivex package).

Issue: SPR-14628

reactor.test.TestSubscriber will not be part of Reactor Core
3.0.0 since it needs to be refactored to fit all the needs
expressed by the users. It is likely to be back later in one
of the Reactor Core 3.0.x releases.

This commit anticipate this removal by temporarily copying
TestSubscriber in spring-core test classes. As soon as
the new TestSubscriber will be available in Reactor Core,
Spring Framework reactive tests will use it again.

DataSourceUtils moved to main core.io.buffer package.
Consistently named Jackson2JsonDecoder/Encoder and Jaxb2XmlDecoder/Encoder.
Plenty of related polishing.

This commit refactors the StringEncoder to a CharSequenceEncoder, in
order to support StringBuilders, Groovy GStrings, etc.

Issue: https://github.com/spring-projects/spring-reactive/issues/120

Issue: SPR-14159

Issue: SPR-14158

Issue: SPR-14492