Fix Protobuf support - HTTP headers already written

Prior to this commit, the `ProtobufHttpMessageConverter` would call
`outputMessage.getBody()` at the beginning of the `writeInternal`
method, thus writing HTTP headers. Since this method is trying to write
"x-protobuf" headers after that, protobuf support wasn't working
properly for the default "x-protobuf" media type.

Thanks Toshiaki Maki for the repro project!

Also fixed:
* improve `MockHttpOutputMessage` behavior to reproduce the read-only
state of HTTP headers once they've been written.

Issue: SPR-12229
Clean up warnings in spring-messaging
Explicitly release rolled-back database savepoints during (long-running) transaction

Issue: SPR-12228
LocalSessionFactoryBean supports JPA 2.1 auto-apply @Converter on Hibernate 4.3

Issue: SPR-12234
Add missing package-info files in spring-test
Update package-info for org.springframework.test
Only release rolled-back database savepoints during managed nested transaction

Issue: SPR-12228
Replaced postProcessFindAllClassPathResourcesResult with doFindAllClassPathResources

Issue: SPR-12231
Avoid use of fragile Swing classes in tests (for compatibility with JDK 8u40)

Issue: SPR-12235
Remove outdated references to ContextLoaderServlet

Issue: SPR-7725
Mention DEFAULT_NONE in @RequestMapping's Javadoc

This change mentions `ValueConstants.DEFAULT_NONE` in @RequestMapping's
Javadoc, in order to explain the default value.

Issue: SPR-9967
Fix "file:///" URL schema in documentation

Issue:  SPR-7735
Reorder list of data access modules in documentation

Issue: SPR-7689
Fix typo in documentation

Issue: SPR-8025
Be defensive adding additional imported configs

Refine the logic introduced in commit 71c6eb2b so that additional
imported @Configuration classes are not considered as candidates if
they have already been parsed.

Issue: SPR-12233