Test case for equal, but not identical, document ID's
Fix grammar in UnitOfWork documentation
Differentiate between ID types in UoW's identity map

Previous use of document IDs as array indexes lead to ambiguity when handling numeric strings. Serializing values to respect ID types will avoid this, although it may be too strict for floats and integers, which are comparable in MongoDB.

Additionally, this change requires that numeric IDs be fetched by the appropriate type, as shown in IdTest::testCollectionId().
Revise documentation for UnitOfWork::getDocumentIdentifier()

The previous documentation looks to have been a copy/paste from ORM.
Fix test for handling of equal, but not identical, IDs