Issue: SPR-13295

This change ensures that an onError outcome from an async request is
also routed to onCompletion handlers registered with
StandardServletAsyncWebRequest.

Issue: SPR-13292

Prior to this commit, the `ResponseStatusExceptionResolver` would use:
* `HttpServletResponse.sendError` if both a status and a reason are set
on the `@ResponseStatus` annotation
* `HttpServletResponse.setStatus` if only a status is set on the
`@ResponseStatus` annotation

This is actually a change of behavior, since this Resolver was using
`sendError` in all cases previously.

Because this change can create issues such as
https://github.com/spring-projects/spring-boot/issues/3623
this commit rollbacks those changes and clarifies the behavior on the
javadoc of the annotation itself.

Issue: SPR-11193, SPR-13226

Issue: SPR-13125

Issue: SPR-13293

Issue: SPR-13037

Issue: SPR-12839

Issue: SPR-13289

SPR-11512 introduced support for annotation attribute aliases via
@AliasFor, requiring the explicit declaration of the 'attribute'
attribute. However, for aliases within an annotation, this explicit
declaration is unnecessary.

This commit improves the readability of alias pairs declared within an
annotation by introducing a 'value' attribute in @AliasFor that is an
alias for the existing 'attribute' attribute. This allows annotations
such as @ContextConfiguration from the spring-test module to declare
aliases as follows.

public @interface ContextConfiguration {

     @AliasFor("locations")
     String[] value() default {};

     @AliasFor("value")
     String[] locations() default {};

    // ...
}

Issue: SPR-13289

Issue: SPR-12970
Issue: SPR-13285

Issue: SPR-13257

Issue: SPR-13286

Issue: SPR-13237

Issue: SPR-13265

Before this change, XhrTransport implementations had to be configured
with the headers to use for HTTP requests other than the initial
handshake.

After this change the handshake headers passed to SockJsClient by
default are used for all other HTTP requests related to the SockJS
connection (e.g. info request, xhr send/receive). A property on
SockJsClient allows restricting the headers to use for other HTTP
requests to a subset of the handshake headers.

Issue: SPR-13254

This commit fixes numerous references to 'legacy' packages within the
Javadoc for the Spring MVC Test framework.

In addition, this commit improves examples in Javadoc, adds links to
related classes where appropriate, and removes unnecessary imports for
types that are only referenced within documentation.

Issue: SPR-13284

Issues SPR-13158

Issue: SPR-13283

Issue: SPR-13267

Issue: SPR-11740
Issue: SPR-13280

Issue: SPR-13271