Issue: SPR-17188

Prior to this commit, when using the `SimpleClientHttpRequestFactory`
as a driver for `RestTemplate`, the HTTP response body would only be
drained if there was an attempt to read it in the first place.

This commit ensures that, even if there's no attempt at reading the
response body, it is properly drained when the response is closed to
make sure that the connection is released in a proper state and can be
put back in the connection pool for reuse.

Issue: SPR-17181

When dealing with `Optional` values in a Controller handler (for
example, values coming from a Spring Data repository), developers might
reuse this code snippet quite often:

```
@GetMapping("/user")
public ResponseEntity<Optional<User>> fetchUser() {
  Optional<User> user = //...
  return user.map(ResponseEntity::ok).orElse(notFound().build());
}
```

This commit adds a new static method on `ResponseEntity` for that,
simplifying the previous snippet with `return ResponseEntity.of(user);`

Note that in case more specific HTTP response headers are required by
the application, developers should use other static methods to
explicitly tell  which headers should be used in each case.

Issue: SPR-17187

Issue: SPR-17129

Includes caching of declared annotation arrays and combined searching for several annotation types (used in SpringCacheAnnotationParser).

Issue: SPR-16933

Issue: SPR-17006

Prior to this commit, if a test class was meta-annotated with multiple
@BootstrapWith declarations that registered different
TestContextBootstrapper implementations, such a configuration would
result in an IllegalStateException, and there was no way to override
this behavior.

This commit addresses this shortcoming by relaxing the explicit
TestContextBootstrapper resolution in BootstrapUtils so that a directly
present @BootstrapWith annotation will now override declarations of
@BootstrapWith that are meta-present. In other words, if @BootstrapWith
is used as a meta-annotation, it can be overridden directly on the test
class via an explicit, local declaration of @BootstrapWith.

Issue: SPR-17006

This reverts commit 7e917335.

This commit handles "empty" cases for `ResponseEntity` controller
handler return types when wrapped with a `java.util.Optional` in Spring
MVC or a single `Publisher` like `Mono`.

Given the following example for Spring MVC:

```
@GetMapping("/user")
public Optional<ResponseEntity<User>> fetchUser() {
	Optional<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

If the resulting `Optional` is empty, Spring MVC will infer a
`ResponseEntity` with an empty body and a 404 HTTP response status.

The same reasoning is applied to Spring WebFlux with Publisher types:

```
@GetMapping("/user")
public Mono<ResponseEntity<User>> fetchUser() {
	Mono<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

This feature is only valid for `HttpEntity` return types and does not
apply to `@ResponseBody` controller handlers.

Issue: SPR-13281

Issue: SPR-17178

Issue: SPR-17180
Issue: SPR-17182

Includes Groovy 2.5.2, Undertow 2.0.12, Hibernate Validator 6.0.12.

Issue: SPR-17129

Rename "Builder" sub-section to "Configuration" and move it in the
beginning before all others since it explains how to create a client
in the first place.

Update content on Reactor Netty connector based on the API in 0.8 and
specifically address Reactor Netty resources and lifecycle.

Issue: SPR-16963

Issue: SPR-16963

Issue: SPR-17120

Prior to this commit, ScriptUtils supported MySQL-style escapes ('\\')
when splitting a script into statements; however, MySQL-style escapes
were not supported when determining if a given script contained a
specified statement delimiter. This caused executeSqlScript() to
erroneously fallback to a newline as the statement separator in such
cases.

This commit fixes this issue by implementing the same check for
MySQL-style escapes in containsSqlScriptDelimiters() that was already
present in splitSqlScript().

Issue: SPR-17120

Its parameter DefaultCacheConfig is just package-visible...

Issue: SPR-16946

Includes revised InstantiationStrategy nullability for args array.

Issue: SPR-17171

Issue: SPR-17174

This commit enables the `-use` javadoc flag so that class usage pages
are included in the aggregated JavaDoc that is published to
https://docs.spring.io/spring-framework/docs/.

Issue: SPR-17173

First step (as proof of concept) toward addressing SPR-17174.

Issue: SPR-17174

When expanding and strictly encoding URI variables, there is no need to
quote `/` and `$` which will be encoded anyway.

Issue: SPR-17168

Issue:SPR-17142

javax.xml.stream.Location is already there as an import there is no need
to use the FQCN in the handleDtd method.

No that JCacheCache extends AbstractValueAdaptingCache instead of
directly implementing Cache an import statement can be used in favor
of using the FQCN for the field and constructor arguments.

Issue: SPR-17169