Includes combined imports of assertion methods and related polishing.

Closes #22889

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).

Issue: SPR-16218

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.

Issue: SPR-15758

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.

Follow-up to:
https://github.com/spring-projects/spring-framework/commit/3d68c496f15c80a75711480a569cda1f7c64dc91

StringDecoder can be created in text-only vs "*/*" mode which in turn
allows a more intuitive order of client side decoders, e.g. SSE does
not have to be ahead of StringDecoder.

The commit also explicitly disables String from the supported types in
Jackson2Decoder leaving it to the StringDecoder in "*/*" mode which
comes after. This does not change the current arrangement since the
the StringDecoder ahead having "*/*" picks up JSON content just the
same.

From a broader perspective this change allows any decoder to deal with
String if it wants to after examining the content type be it the SSE
or another, custom decoder. For Jackson there is very little value in
decoding to String which works only if the output contains a single
JSON string but will fail to parse anything else (JSON object/array)
while StringDecoder in "*/*" mode will not fail.

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.

Issue: SPR-15350

All method argument resolvers now explicitly check for the presence
of a reactive type wrapper and reject it where not expected.

Issue: SPR-15297

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

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

Issue: SPR-14542

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

Issue: SPR-14843

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 commit removes the usage of Reactor adapters (about to
be moved from Reactor Core to a new Reactor Adapter module).
Instead, RxReactiveStreams is now used for adapting RxJava
1 and Flowable methods are used for RxJava 2.

Issue: SPR-14824

This commit replaces direct use of Validator and ConversionService in
the reactive @RequestMapping infrustructure in favor of using the
BindingContext.

Issue: SPR-14541

This commit adds a BindingContext to be used in spring-web-reactive
@RequestMapping infrastructure (comparable to WebDataBinderFactory in
spring-web-mvc) for access to the default model, data binding,
validation, and type conversion purposes.

Issue: SPR-14541

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.

Issue: SPR-14159

Issue: SPR-14492

Reactor's `DependencyUtils` has been renamed to `Converters` and
all the `from` converter methods have been disambiguated to
`fromPublisher`, `toPublisher`.