Do not overwrite get_session of base class

I encountered this problem while working with a custom base class. Since I define all my behaviour in it and directly subclass from the `ModelForm` the factory spits out, I created `get_session` there but this code overwrote it.

All tests pass on my end. Not familiar enough with the code base to determine if I should add a test for this. If a test should be added, please tell me where, as I just started using your library.

Note that this relies on the assumption that only a single base class is used (which is true for the current code, but I don't know about future plans).