DATAMONGO-893 - Convertor must not write "_class" information for know types.

The fix for DATAMONGO-812 enforced proper writing of _class information for Updates. This caused trouble when querying documents by nested (complex) properties using an 'in' clause.

We now actively pass on property type information to MetadataBackedField to ensure type hints get picked up correctly when converting a value to the according DBObject.