Fix a couple problems with FACTORY_DJANGO_GET_OR_CREATE

I wanted to do this:

```
class SomethingFactory(factory.DjangoModelFactory):
    FACTORY_DJANGO_GET_OR_CREATE = ['pk', 'id']
```

so I could call with either id= or pk= and have it work, or call with neither and have that work too (like a normal factory).  These patches make both of these scenarios work.