DATAREST-221 - Further refinements in Projector.

Prevent NullPointerException in case we don't find a requested projection. We simply return the source object if so. Added unit tests to make sure we don't introduce a regression.

Removed NoOpProjector from production code as it is only needed in test cases anyway.
DATAREST-261 - Fixed RepositoryRelProvider registration.

We now register a RepositoryRelProvider that hadn't been exported before so that clients using a RelProvider will automatically see potentially customized rels for repositories.

Changed RepositoryRelProvider to lazily depend on the ResourceMappings as will be requested eagerly for injection into the configuration class itself.