- Add "Processing" section (also replaces Advanced Customizations)
- Add information on out-of-the-box behavior
- Add more deails on @CrossOririn default configuratio
- Add cross-references between Spring MVC and WebFlux
- Polish

1. setComplete checks the isCommitted flag to avoid an unnecessary
debug message. This method is meant to be safe to call many times.

2. setStatusCode lowers log message to TRACE, since the return value
communicates the outcome it's arguably much less critical.

3. Add comment and test case for ResponseStatusExceptionHandler.
A ResponseStatusException is clearly meant to be handled by this
handler so don't let it pass through even if the respones is
committed.

Issue: SPR-16231

Issue: SPR-16225

Issue: SPR-16194

Issue: SPR-16230

Issue: SPR-16229

Before this commit, the return value from write was interpreted as the
data being fully written and ready to be released via releaseData().

This is not true for WebSocketSession implementations where a true
return value simply means the message was sent with the full payload
but releas is not appropriate until a send confirmation.

Technically not an issue since WebSocketSession's extending this do
not use pooled buffers. Nevertheless this commit refines the semantics
of write, removes the releaseData() method, and makes sub-classes
responsible for releasing the buffer when fully written (and they
know best when that is). As a bonus currentData is now private.

Issue: SPR-16207

After this commit, Tomcat and Undertow WebSocketSession imlpementations
start out in suspended mode and wait for demand.

The JettyWebSocketSession is capable of suspending but it doesn't seem
to work if invoked before any messages are received. That may become an
issue if there is a case where no demand appears long enough for more
messages to accumulate than we can hold.

UnderowServerHttpRequest would ideally also start in suspended mode but
that also doesn't work. It is not an issue in this case since we can
ignore the read notifications.

Servlet API requires a proactive check before it calls you back so
there is no need to suspend.

Issue: SPR-16207

This commit turns suspendReading() into a readingPaused() notification
that is invoked after a succession of reads stops because there is no
more demand. Sub-classes can use this notification to suspend, if that
applies to them.

Most importantly the notification is guaranteed not to overlap with
checkOnDataAvailable() which means that suspend does not need to be
atomic and guarded against resume. The two can and do compete all the
time when reading ends with no demand, and a request for demand arrives
concurrently.

Issue: SPR-16207

Issue: SPR-16224

Access-Control-Allow-Credentials CORS header, used to
allow cookies with CORS requests, is not set to true
anymore by default when enabling CORS with
@CrossOrigin or global CORS configuration in order to
provide a more secured default CORS configuration.

The related allowCredentials property now requires to
be set to true explicitly in order to support cookies
with CORS requests.

Issue: SPR-16130

Issue: SPR-16226

Issues: SPR-16207

Issue: SPR-16207

Issue: SPR-16207

Transition from DEMAND->NO_DEMAND:
Two concurrent threads enter DEMAND.request and DEMAND.onDataAvailable.
And DEMAND.onDataAvailable finishes before DEMAND.request to be able to
update the demand field then a request for reading will be lost.

Transition from READING->NO_DEMAND:
readAndPublish() returns false because there is no demand but before
switching the states READING.request is invoked again a request for
reading will be lost.

Changing READING->DEMAND/NO_DEMAND is made conditional so that the
operations will be executed only if changing states succeeds.

When in READING state detect completion before each next item in order
to exit sooner, if completed.

Issue: SPR-16207

- Ensure completion signal (normal/exception) will be delivered to
the subscriber when transition from UNSUBSCRIBED->COMPLETED

- According to the specification "Publisher.subscribe MUST call onSubscribe
on the provided Subscriber prior to any other signals to that Subscriber" so
ensure onComplete/onError signals will be called AFTER onSubscribe signal.

Issue: SPR-16207

Issue: SPR-16207

Issue: SPR-16217

* pr/1601:
  Fix copy/paste in Javadoc

Closes gh-1601

Issue: SPR-16218

Issue: SPR-16219