DATAMONGO-1086 - Mapping fails for collection with two embbeded types that extend a generic abstract.

We now use the type information of the raw property type to check if we need to include _class.
DATAMONGO-1100 - Upgrade to new PersistentPropertyAccessor API.
DATAMONGO-943 - Add support for $position to Update $push $each.

Prepare issue branch.
DATAMONGO-943 - Add support for $position to Update $push $each.

We now support $position on update.push.
DATAMONGO-943 - Add support for $position to Update $push $each.

Corrected error message. Added test case for null position.