ChoiceFields were not populated with the correct value.

I noticed when using 0.3.1 the choicefields of my forms where not set to the correct value when passing instance=SomeModelInstance to the form. I examined the changes I made previously and compared them to the current master, and applied a fix.

All py.tests are passing, however I did change the model_admin test. Please review the changes with care before/if merging them in :).