- formatting
- code style
- organized imports
- precondition assertions
- suppressed warnings
- Javadoc enhancements

Issue: SPR-13158

This is necessary to ensure that the version of HtmlUnit that we use is
compatible with HttpComponents 4.5.

Issue: SPR-13158

Issue: SPR-13158

Issue: SPR-13158

This commit introduces integration between MockMvc and HtmlUnit, thus
simplifying end-to-end testing when using HTML-based views and enabling
developers to do the following.

 - Easily test HTML pages using tools such as HtmlUnit, WebDriver, & Geb
   without the need to deploy to a Servlet container

 - Test JavaScript within pages

 - Optionally test using mock services to speed up testing

 - Share logic between in-container, end-to-end tests and
   out-of-container integration tests

Issue: SPR-13158

Since @TransactionConfiguration is now deprecated, this commit also
deprecates TransactionConfigurationAttributes with the intention of
completely removing TransactionConfigurationAttributes once
@TransactionConfiguration has been removed.

Issue: SPR-13277

Due to common usage of @Rollback(false), this commit introduces a new
@Commit annotation that more clearly conveys the intent of the code
while retaining the run-time semantics.

@Commit is in fact meta-annotated with @Rollback(false).

Issue: SPR-13279

Issue: SPR-13276, SPR-13277

Since Spring Framework 2.5, @Rollback has been supported on test
methods, with class-level rollback settings configured via
@TransactionConfiguration; however, allowing @Rollback to be declared
on test classes with method-level declarations overriding class-level
declarations would prove more intuitive than having to declare both
@TransactionConfiguration and @Rollback. Furthermore, the
transactionManager flag in @TransactionConfiguration was made
superfluous many years ago with the introduction of support for a
qualifier in @Transactional.

This commit enables @Rollback to be declared at the class level for
default rollback semantics within test class hierarchies and deprecates
@TransactionConfiguration in favor of @Rollback and @Transactional
qualifiers.

Issue: SPR-13276, SPR-13277

Issue: SPR-13272

AbstractMessageConverter provides overloaded methods with a conversion hint, MappingJackson2MessageConverter takes that hint into account, and SimpMessagingTemplate transformes such a hint in the given headers map into an explicit argument invocation argument.

Issue: SPR-13265

Prior to this change, calling the `setDateHeader` method on a
Spring Test MockHttpServletResponse instance would just store the given
long value in a Map, not writing it as a formatted date String.
Also, calling `getDateHeader` on a MockHttpServletRequest would not
support date strings and could not parse those values.

This can be problematic when testing features related to date headers
such as "Expires", "If-Modified-Since", "Last-Modified", etc.

This commit adds formatting and parsing capabilities to Servlet Mocks
for date strings in HTTP headers.

When formatting dates to Strings, the date format used is the one
preferred by the HTTP RFC. When parsing date Strings, multiple date
formats are supported for better compatibility.

Issue: SPR-11912

SPR-13211 introduced support for reusing mock requests in Spring MVC
Test if the request was created by the the Spring TestContext
Framework. Unfortunately, that change makes it impossible for
MockMvc.perform() to be invoked multiple times within the same test
method without side effects. For example, session attributes and
request parameters are transparently and unexpectedly retained for
subsequent invocations of perform(), causing certain categories of
tests to fail.

This commit reverts the changes introduced in SPR-13211 and introduces
a new MockMvcReuseTests class to serve as regression tests within
Spring's test suite.

Issue: SPR-13260, SPR-13211

Issue: SPR-13257

HTTP headers such as "Expires", "Last-Modified" all use date
strings like "Tue, 21 Jul 2015 10:00:00 GMT". Prior to this commit,
there was no way to match those header values, besides formatting dates
manually.

This commit introduces a new HeaderResultMatcher to test those date
headers using a long timestamp:

```
this.mockMvc.perform(get("/persons/1").header("If-Modified-Since", now))
  .andExpect(status().isNotModified())
  .andExpect(header().dateValue("Last-Modified", timestamp));
```

Issue: SPR-13263

Streamlined WebContentGenerator API variants: checkRequest, prepareResponse, applyCacheControl, applyCacheSeconds

Issue: SPR-11792

Issue: SPR-13006