MockMvc now properly detects the presence of an AsyncContext and
re-uses the response instance used to start it.

This commit also includes a minor fix in
ResponseBodyEmitterReturnValueHandler to ensure it does not disable
ETag related content buffering for reactive return values that do not
result in streaming (e.g. single value or collections).

Issue: SPR-16067

MockServerWebExchange now has a second factory method that accepts a
MockServerHttpRequest.BaseBuilder.

Issue: SPR-16079

Issue: SPR-16082

* pr/1566:
  Missing some double backticks with apostrophes

Closes gh-1566

Issue: SPR-16052

Issue: SPR-16076
(cherry picked from commit 17fb4fed)

Issue: SPR-16078

(cherry picked from commit dff2c84e)

Issue: SPR-16030

Issue: SPR-16049

Issue: SPR-16075

Issue: SPR-16073

Issue: SPR-16066

Issue: SPR-16059

Issue: SPR-16066

Includes redeclaration of CodecConfigurer.DefaultCodecs without concrete Jackson decoder/encoder references in order to avoid hard Jackson dependency.

Issue: SPR-16064

Before this commit, when adding filters to a builder obtained via
`WebClient.mutate()`, the filters were added both to the original client
as well as the mutated builder. This commit fixes that.

Issue: SPR-16059

Issue: SPR-16055

Issue: SPR-16063

Properly separate tocbot from asciidoctor styles and use main.css to
include both.

Prior to this commit, the `ResourceWebHandler` would itself handle the
response with an HTTP 404 in many cases, including a missing static
resource.

This does not give a chance to `WebExceptionHandler` instances to handle
that error and, for example, display an error page.

See spring-projects/spring-boot#8625

Issue: SPR-16023

Prior to this commit, the Resource `HandlerMapping` configured by
`@EnableSpringMvc` would be set up with only one custom
`HandlerInterceptor` (the one exposing the `ResourceUrlProvider` for
resolving URLs of static resources).

This behavior is not consistent with the rest of the default
configuration which leverages the interceptors configured against the
interceptor registry (with possible user-provided ones).

This change allows to register interceptors against resource handling;
this can be useful for metrics purposes, as in
spring-projects/spring-boot#10335.

Issue: SPR-16034

Easier to find (at the top level) and better summary of testing support.
Improve structure headings names.
Remove or update outdated content.

Issue: SPR-14579