UnitOfWork incorrectly flushes certain documents with embedded documents on first flush

With documents that have embedded documents, the changeset is sometimes calculated incorrectly so that unmodified documents are flushed unnecessarily. This is similar in spirit to #MODM-83, but this isn't covered by that fix.

This can be seen when the document has an embedded document that uses a discriminator map. When calculating the changeset, the original value will have the discriminator field and value present while the actual value does not contain the discriminator field.

Covering tests with fixes to follow shortly.