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

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.

This has to be done as the fix for DATAMONGO-812 enforced proper writing of _class information for Updates, which caused trouble when querying documents by nested (complex) properties using an 'in' clause.

Original pull request: #169.