DATAMONGO-893 - Mapping Convertor does not remove "_class" property on collection of embedded objects.

Class type information is now removed correctly for nested objects.
We now retain the property type information when constructing a new MetadataBackedField for a keyword, e.g. $in, from a given MetadataBackedField. 
This makes sure that we have correct type information available for the conversion in MappingMongoConverter.