Issue: SPR-16203

Issue: SPR-16385

Issue: SPR-16407

Issue: SPR-16365

Issue: SPR-16210

This commit makes sure there is no custom Content-Disposition header
before setting one automatically.

This commit also adds a headers(Consumer<HttpHeaders>) method, so that
one can user the nicer methods of HttpHeaders, as opposed to basic
strings.

Issue: SPR-16376

Issue: SPR-16267

Issue: SPR-16404

Since Java7 HttpURLConnection offers setFixedLengthStreamingMode method with long parameter which should be prefered over version with int argument, therefore casting ContentLength to int is no longer needed. Moreover it makes impossible to stream payload larger than Integer.MAX_VALUE

* pr/1647:
  Polish InterceptorRegistration

Closes gh-1647

Now that SPR-16307 makes it possible to write a part with a Publisher
we need to ensure we get the filename correctly when writing with
Publisher<Resource>.

Issue: SPR-16376

Issue: SPR-16275

After this commit DefaultUserDestinationResolves no longer looks at
whether AntPathMatcher is configured with "." as separator and rather
expects to be explicitly told whether to keep the leading slash in
translated destinations which actually depends on what the message
broker supports (e.g. RabbitMQ "/", Artemis ".") or how it is
configured (simple broker could be either way).

There is also a minor improvement in SimpMessagingTemplate to ensure
user destinations are correctly formed based on what the
DefaultUserDestinationResolver expects. When using "." as separtor it
allows sending messages to "queue.q1" rather than "/queue.q1".

Issue: SPR-16275

The previous fix #09f1f7 did not actually address the issue but only
moved it further down, so instead of the subscribe(), it was consuming
it inside the MultipartHttpMessageWriter#write method which returned
this.body.then(), and then again for the actual request body writing.

In this commit MultipartHttpMessageWriter#write returns Mono.empty()
since we don't actually want to write the part content from there, but
only want to access it as soon as it is availabele, for writing to
the request body.

Issue: SPR-16402

Fixed by creating `Callable`-based variants, as explained in the JIRA
issue.

Issue: SPR-16403

Issue: SPR-16402

Issue: SPR-16210

Issue: SPR-15624

Issue: SPR-15624

This commit checks the Etag/LastModified headers on the incoming
request, and sets a 304 Not Modified status with no body when they
match, by delegating to ServerWebExchange.checkNotModified.

Issue: SPR-16348

Issue: SPR-16369

This commit separates the creation of a response so that it is executed
after the synchronized block inside which requests need to be matched
and validated (for order and count).

This allows a ResponseCreator to be slow or block if it has to.

Issue: SPR-16319

Issue: SPR-16369

* pr/1646:
  Fix doc typo

Closes gh-1646

* pr/1645:
  Fix doc typo

Closes gh-1645