DATAMONGO-1070 - Fixed a few glitches in DBRef binding for repository query methods.

The QueryMapping for derived repository queries pointing to the identifier of the referenced document. We now reduce the query field's key from reference.id to reference so that the generated DBRef is applied correctly and also take care that the id's are potentially converted to ObjectIds. This is mainly achieved by using the AssociationConverter pulled up from UpdateMapper in ObjectMapper.getMappedKey().

MongoQueryCreator now refrains from translating the field keys as that will fail the QueryMapper to correctly detect id properties.

Fixed DBRef handling for StringBasedMongoQuery which previously didn't parse the DBRef instance created after JSON parsing for placeholders.