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](https://jira.spring.io/browse/DATAMONGO-812) enforced proper writing of `_class` information for `Updates` which caused trouble when querying documents by nested (complex) properties using an `$in` clause.