DATAMONGO-724 - Added test case to show registered converters work.

Added test case that shows that if a custom converter doesn't write type information on its own, the managed type can't be used in polymorphic scenarios. Direct type mappings still work as expected.
DATAMONGO-724 - Adapted test case to show type information written by converters gets regarded.
DATAMONGO-721 - Updates retain type information for collections properties.

Added test case to demonstrate that the fix for DATACMNS-345 will propagate into Spring Data MongoDB.

Original pull request: #52.