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

Prior to this change, calling the `setDateHeader` method on a
MockHttpServletResponse instance (internal implementation for testing
the spring-web module) would just store the given long value in a Map,
not writing it as a formatted date String.

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

This commit formats long dates into date Strings using the date format
recommended by the RFC and the GMT time zone.

As filter-based libraries and projects (such as Spring Security) may
use the "Pragma" header in HTTP responses, WebContentGenerator should
make sure that such headers are overwritten to avoid clashes with
the HTTP caching headers set by the HTTP caching configuration.

Issue: SPR-13252

Use ServletHttpResponse.setDateHeader whenever possible and avoid using
SimpleDateFormat.

Prior to this commit, the Spring MVC Test Framework always created a
new MockHttpServletRequest, disregarding any mock request already
present in Spring Web's RequestContextHolder -- for example, one
created by the ServletTestExecutionListener in the Spring TestContext
Framework (TCF).

This commit modifies MockHttpServletRequestBuilder so that it reuses a
mock request created by the TCF. However,
MockMultipartHttpServletRequestBuilder continues to always create a new
MockMultipartHttpServletRequest since a MockHttpServletRequest created
by the TCF is not directly compatible with a
MockMultipartHttpServletRequest. Furthermore, in order to avoid
unforeseen side effects, MockHttpServletRequestBuilder will always
create a new MockHttpServletRequest if a mock request is present in the
RequestContextHolder but not created by the TCF.

Issue: SPR-13211

This commit replaces calls to System.out.println() with explicit logging.

Issue: SPR-13247

Issue: SPR-13236

Issue: SPR-13192

Issue: SPR-13248

Issue: SPR-13243