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

Issue: SPR-15365

Issue: SPR-15390

Issue: SPR-15379

Issue: SPR-15384

Issue: SPR-15395

This commit adds support for reactive library types to be returned
directly from controller methods adapting them either to a
ResponseBodyEmitter (streaming) or DeferredResult (non-streaming).

The reactive libraries supported are the ones that can adapted to a
Reactive Streams Publisher through the ReactiveAdapterRegistry.

Issue: SPR-15365

Revise Javadoc on AsyncHandlerMethodReturnValueHandler to clarify its
main purpose is to prioritze custom async return value handlers ahead
of built-in ones. Also replace the interface from built-in handlers
which are prioritized already.

Remove DeferredResultAdapter and ResponseBodyEmitterAdapter --
introduced in 4.3 for custom async return value handling, since for
5.0 we will add built-in support for reactive types and the value of
these contracts becomes very marginal.

Issue: SPR-15365

* pr/1370:
  Polish javadoc

Closes gh-1370

Issue: SPR-15361

This change allows the functional WebFlux API to support natively
reactive types and also makes it possible for View implementations to
disable async attributes resolution if they want for example take
advantage of stream rendering.

It also makes AbstractView#getModelAttributes() asynchronous.

Issue: SPR-15368

Replace the overloaded "retrieve" methods with a single retrieve() +
ResponseSpec exposing shortcut methods (bodyToMono, bodyToFlux)
mirroring the ClientResponse shortcuts it delegates to.

Unlike exchange() however with retrieve() there is no access to other
parts of ClientResponse so ResponseSpec exposes additional shortcuts
for obtain ResponseEntity<T> or ResponseEntity<List<T>>.

Issue: SPR-15294