Issue: SPR-15418

The java.util.Optional wrapper should not affect the support for
"request param" arguments with or without the annotation as it
works on the Spring MVC side.

Issue: SPR-15211

Previously ForwardedHeaderFilter would return the same StringBuffer for every invocation. This
meant that users that modified the StringBuffer changed the state of the HttpServletRequest.

This commit ensures that a new StringBuffer is always returned for ForwardedHeaderFilter.

Issue: SPR-15423

Issue: SPR-15417

Issue: SPR-14512

Issue: SPR-15409

This commit uses the newly introduced
`PathPattern.getPathRemaining(String)` in the functional web framework.
With this change, all path predicates can be used for nested router
functions, so the `pathPrefix` predicate is no longer required and has
been removed.

Issue: SPR-15336

With this change there is a new getPathRemaining() method on
PathPattern objects. It is called with a path and returns
the path remaining once the path pattern in question has
matched as much as it can of that path. For example if the
pattern is /fo* and the path is /foo/bar then getPathRemaining
will return /bar. This allows for a set of pathpatterns
to work together in sequence to match a complete entire path.

Issue: SPR-15336

This commit deletes images that are no longer used in the Reference
Manual.

Issue: SPR-14997

This commit uses a tip provided by @mojavelinux to set the syntax
highlighter to Rouge for the PDF version of the User Guide.

Once Asciidoctor 1.5.7 has been released we will likely switch to Rouge
for the HTML version of the Reference Manual as well.

Issue: SPR-14997

Issue: SPR-14512

Issue: SPR-15411

Issue: SPR-15411

Issue: SPR-15387

Includes upgrade to HtmlUnit 2.26 and Selenium 3.3.1.

Issue: SPR-14512

Issue: SPR-14512

Includes dependency updates (JavaMail 1.6 RC1, RxJava 2.0.8, Tomcat 8.5.13, Undertow 1.4.12, XNIO 3.4.5)

Issue: SPR-14512

This commit introduces support for the server-side methods on
HttpMessageReader and HttpMessageWriter. It does so by introducing an
Optional ServerHttpRequest in BodyInserter.Context, and an Optional
ServerHttpResponse in BodyExtractor.Context. On the client-side, these
optionals return Optional.empty(); on the server-side, they return the
respective server-side messages.

Issue: SPR-15370

InvalidDefinitionException has been introduced in Jackson 2.9 to be
able to differentiate invalid data sent from the client (should still
generate a 4xx HTTP status code) from server side errors like beans with
no default constructor (should generate a 5xx HTTP status code).

Issue: SPR-14925

This commit converts interlaced PNGs to standard PNGs and uses PNGs
instead of GIFs consistently within the reference manual in order to
avoid issues with AsciiDoc’s support for inlined images.

Issue: SPR-14997

Issue: SPR-15365

Related to SPR-15412

- ServletServerHttpResponse.ResponseAsyncListener#onError/onTimeout
must complete the async operation
- ServletHttpHandlerAdapter.HandlerResultSubscriber#onComplete must
check that the async operation is not completed

Issue: SPR-15412

Issue: SPR-15410