Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:

* An unsaved object -- This actually doesn't generate the same failure
  but another ORM-level exception. The test verifies it's the case.

* An instance of a model with a __nonzero__() method thant returns False
  for it. This doesn't fail because that code path isn't executed.

* An instance of a model with a CharField PK and an empty value for it.
  This doesn't fail.