DATAREST-226 - After release cleanups.

Upgraded to Spring Data snapshot dependencies. Added missing changelog entries.
DATAREST-50 - Added test case demonstrating this issue no longer exists.

Properties with null values properly get rendered in JSON outputs. This was fixed some time ago. Added test case to prove it.

Original pull request: #132.
DATAREST-246 - ValidatingRepositoryEntityListener now uses ObjectFactory<Repository>.

ValidatingRepositoryEntityListener previous referred to a Repositories instance which causes a circular reference on initialization after some changes in Spring Data Commons Repositories.

Major cleanups in the ValidatingRepositoryEntityListener implementation.