* pr/1158:
  Fix typo

Closes gh-1158

Since SPR-14522, the web reactive framework supports checkNotModified
features. This commit aligns the existing MVC infrastructure with
web reactive's behavior.

Because of the new Servlet 3.0 baseline, some constraints
aren't relevant anymore and duplicate code has been removed in
`HttpEntityMethodProcessor`.

Issue: SPR-14659

Issue: SPR-14521

A straight-forward port of the resource handling support in
spring-webmvc to spring-web-reactive. Primarily adapting contracts and
implementations to use the reactive request and response and the
reactive ResourceHttpMessageWriter.

Issue: SPR-14521

Issue: SPR-5863

Issue: SPR-5863

Issue: SPR-4365

Whereas the existing TestContextConcurrencyTests verify support for
concurrency in the TestContextManager and TestContext, this commit
introduces SpringJUnit4ConcurrencyTests that verify support for
concurrent test execution in Spring's JUnit 4 support, namely in the
SpringRunner, SpringClassRule, and SpringMethodRule.

The tests executed by this new test class come from a hand-picked
collection of test classes within the test suite that is intended to
cover most categories of tests that are currently supported by the
TestContext Framework on JUnit 4.

Note, however, that the chosen test classes intentionally do not
include any classes that fall under the following categories.

 - tests that make use of Spring's @DirtiesContext support

 - tests that make use of JUnit 4's @FixMethodOrder support

 - tests that commit changes to the state of a shared in-memory database

Issue: SPR-5863

This commit improves the exception message thrown when a test's
ApplicationContext is no longer active by explaining that the cause
may be due to parallel test execution.

Issue: SPR-5863

Issue: SPR-5863

As per specification "The Subscription MUST allow the Subscriber to
call Subscription.request synchronously from within onNext or
onSubscribe". With the current implementation if Subscription.request
is called more than once when Subscriber.onSubscribe ISE will be
thrown - java.lang.IllegalStateException: DEMAND.
With this fix the implementation will not throw ISE and will allow
many invocations of Subscription.request when
Subscriber.onSubscribe.

This commit fixes `ResponseEntityResultHandler` so that it only tries to
call `writeBody` if the `ResponseEntity` is not null. In case the
response entity body is null, the response is flushed right away and the
request is signaled as handled.

Issue: SPR-14663

Prior to this commit, executing tests concurrently in the TestContext
Framework (TCF) was unsupported and typically lead to unpredictable
results.

This commit addresses this core issue by supporting concurrent
execution in the TestContextManager and the DefaultTestContext.

Specifically, the TestContextManager now uses ThreadLocal storage for
the current TestContext, thereby ensuring that any registered
TestExecutionListeners and the TestContextManager itself operate on a
TestContext specific to the current thread.

In order to avoid repeatedly incurring the costs of the overhead of the
TCF bootstrapping process, the original TestContext built by the
TestContextBootstrapper is used as a template which is then passed to
the copy constructor of the concrete implementation of the TestContext
to create the context for the current thread. DefaultTestContext now
implements such a copy constructor, and all concrete implementations of
TestContext are encouraged to do the same.

If the TestContext built by the TestContextBootstrapper does not
provide a copy constructor, thread-safety and support for concurrency
are left completely to the implementation of the concrete TestContext.

Note, however, that this commit does not address any thread-safety or
concurrency issues in the ContextLoader SPI or its implementations.

Issue: SPR-5863

This commit inlines the basic implementation of AttributeAccessorSupport
and converts the LinkedHashMap to a ConcurrentHashMap.

In addition, attributes are now included in toString().

Issue: SPR-5863

Issue: SPR-14421

This change puts the MockHttpServerRequest/Response under
org.springframework.mock.http.server.reactive.test which
mirrors the arrangement we have with the Servlet mock equivalents
and sets up the addition of MockHttpServerRequest/Response in
spring-test.

Issue: SPR-14421

Issue: SPR-14421

Remove stray ` in import-into-idea.md

Changes, according the latest information.

This commit documents the regexp support in `AntPathMatcher` when
matching for URL patterns. This support is also mentioned in places
where developers can register patterns for ViewControllers or resource
handlers.

Issue: SPR-14652

Issue: SPR-14522

Issue: SPR-14522

Issue: SPR-14522

Issue: SPR-14658

This commit avoid merging automatically the SSE stream with
Flux.never(). Since browsers automatically reconnect when
the HTTP connection is closed, it is now the user responsability
to optionally perform a concatWith(Flux.never()) on streams that
complete in order to avoid receiving the data multiple times on
client side.

The behavior with hot streams that never complete does not change.

Issue: SPR-14578

* pr/1151:
  Fix class literal in instanceof