Failing test for DDC-2306

As of DDC-1734, proxies should have null identifier when the UnitOFWork
refreshes entities and replaces them (marking them as un-managed).
The problem here is that refreshes on entities with same identifier
but different type are still considered same, and the UnitOfWork discards
the proxy instance instead of ignoring it.
Fixing DDC-2306
Merge pull request #585 from Ocramius/hotfix/DDC-2306

Hotfix/DDC-2306