HttpHeaders fails getAllow if set to EmptyCollection

Prior to this commit, calls to getAllow would fail is setAllow was set
to an EmptyCollection right before.

    java.lang.IllegalArgumentException: No enum constant
    org.springframework.http.HttpMethod

This commit fixes this by testing the header value for an empty value
before trying to use it to get a value from the Enum.

Issue: SPR-11917
Spring's default JPA persistence unit exposes package metadata as well (currently only supported with Hibernate)

Issue: SPR-10910
Introduced AnnotationConfigRegistry as common interface for AnnotationConfig(Web)ApplicationContext

Issue: SPR-11814
Polishing
Latest dependency updates (AspectJ 1.8.1, Reactor 1.1.2, Tomcat 8.0.9, Jackson 2.4.1, JAMon 2.76)
Delete accidental call to System.err
Re-enable OpenJPA + AspectJ tests

With the upgrade to JDK 8, the OpenJPA/AspectJ integration tests now
pass again. Thus, we are re-enabling these tests.

Issue: SPR-10333
Add TODO for fate of Hibernate JPA integration tests

Issue: SPR-11922