DATAMONGO-972 - Querydsl integration now handles references correctly.

SpringDataMongodbSerializer now overrides the necessary methods to create the appropriate DBRef objects when serializing data via Querydsl.

We currently disable the test case as it the fix taking effect requires Querydsl 3.4.1 which unfortunately breaks Java 6 compatibility. We include the fix nonetheless to allow users on Java 7 to potentially use the latest Querydsl.

Original pull request: #203.
Related tickets: querydsl/querydsl#803.