* server_sent_event:
  Refactored SseEvent to ServerSentEvent

 - Renamed SseEvent to ServerSentEvent to make the name less redundant.
 - ServerSentEvent is now immutable, having a builder to create new instances.
 - Realigned the class properties to more closely match the events
   described in the spec, so that `reconnectTime` becomes `retry`, and
   `name` becomes `event`.

Replace manual synchronization block in SpringClassRule with Java 8's
Map.computeIfAbsent().

Issue: SPR-12421

Operators.getAndSub was removed in Reactor 3.0.1, this commit copies the
implementation over to AbstractRequestBodyPublisher, which used it.

This commit introduces integration tests that verify the expected
behavior for @Nested tests in conjunction with the SpringExtension for
JUnit Jupiter, including automatic application of
TestExecutionListeners to the enclosing test instance of a @Nested
test instance.

Issue: SPR-14150

Due to restrictions imposed by JUnit 4, the SpringClassRule must be
declared as a public static field, which makes it impossible to be
declared directly within a nested (i.e., inner) test class.

This commit demonstrates a work-around that makes it possible to use
the SpringClassRule and SpringMethodRule in a nested (i.e., inner) test
class when using a custom JUnit 4 runner such as the
HierarchicalContextRunner from Stefan Bechtold.

The trick is to have inner test classes extend a class that properly
declares the SpringClassRule and SpringMethodRule. The
SpringRuleConfigurer in this commit serves as an example.

Note, however, that each such nested test class must declare its own
@ContextConfiguration. Furthermore, TestExecutionListeners in the
Spring TestContext Framework are not applied to the enclosing instance
of such an inner test class, meaning that @Autowired fields in the
enclosing instance will not be injected, etc.

Issue: SPR-14150

A little terminiology first:

* merge.from - a project that contains source that will be merged
               into merge.into
* merge.into - a project that contains source code that will have
               code from merge.from merged into it.

Previously a module that dependended on merge.into would not see
the merge.from module as a transitive dependency. This worked fine
from a Gradle build because all the code from merge.from is merged
into the merge.into jar. However, in an IDE it did not work because
the IDE does not assemble a jar.

This fix ensures that merge.from modules are automatically added
to the classpath of any module relying on the merge.into project.

Fixes SPR-14650

This commit picks up where SPR-14614 left off by introducing a new
@EnabledIf annotation to serve as a logical companion to @DisabledIf.

In addition, this commit extracts common logic from DisabledIfCondition
into a new AbstractExpressionEvaluatingCondition base class which the
new EnabledIfCondition also extends.

An @EnabledOnMac annotation is also included in the Javadoc as well as
in the test suite to demonstrate support for custom composed annotations.

Issue: SPR-14644

Issue: SPR-14648

Issue: SPR-14646

Issue: SPR-14645

Issue: SPR-14577

(cherry picked from commit 37670924)

Issue: SPR-14635

Issue: SPR-14642

This commit updates the instructions on getting started with
Spring Web Reactive and also updates constructors and setters to
streamline the getting started procedure.

Issue: SPR-14640

This is a follow-up on commit 3b95e0b6 relaxing the expectation that a
ServletContext is present. Instead we check defensively and fall back
on PathExtensionContentNegotiationStrategy which can use JAF.

Issue: SPR-14577

Update tutorial for import and build project in IntelliJ IDEA 2016.2.2. Steps are outdated and existing issues is not applicable in new versions.

Create an explicit heartbeat task with an experiration flag so that
it can be cancelled reliably vs relying on the ScheduledFutureTask
cancel method which may return true even if the task is already
running.

Issue: SPR-14356

Issue: SPR-14638

Prior to this commit, exceptions thrown by the
`HandlerMethodArgumentResolver` would be wrapped into
`IllegalStateException`s.

This commit makes sure that a DEBUG log is written with the relevant
information and that the root cause is not wrapped into another
exception, so that the appropriate `ExceptionHandler` can be used to
deal with this error.

Issue: SPR-14618

This commit adds Smile and CBOR Jackson HttpMessageConverters
and make it possible to create Smile and CBOR ObjectMapper via
Jackson2ObjectMapperBuilder, which now allows to specify any
custom JsonFactory.

Like with JSON and XML Jackson support, the relevant
HttpMessageConverters are automaticially configurered by
Spring MVC WebMvcConfigurationSupport if jackson-dataformat-smile
or jackson-dataformat-cbor dependencies are found in the classpath.

Issue: SPR-14435

This commit upgrades the SonarQube plugin so that it is compatible with
Gradle 3.

Issue: SPR-14569

Issue: SPR-14568

Issue: SPR-14577

Issue: SPR-14586

Prior to this commit, the DisabledIfCondition did not trim whitespace
from expressions configured via @DisabledIf. Consequently, results such
as "  true  " would evaluate to "false".

This commit fixes this problem by trimming all expressions configured
via @DisabledIf.

Issue: SPR-14614

This commit introduces tests for DisabledIfCondition that verify actual
condition evaluation results and exception handling; whereas, the
existing tests in DisabledIfTestCase only test the "happy paths" and
standard cases.

Issue: SPR-14614

This commit ensures that a user provides a non-empty expression in
declarations of @DisabledIf.

Issue: SPR-14614

Issue: SPR-14636