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.
* Renamed the __unicode__ methods
* Applied the python_2_unicode_compatible decorator
* Removed the StrAndUnicode mix-in that is superseded by
python_2_unicode_compatible
* Kept the __unicode__ methods in classes that specifically
test it under Python 2
