-Change: Move get_db_prep_save to before value checking.

This means that, while the fields now have to check for None values in
get_db_prep_save, those same fields can now save values that would
evaluate to False (False, '', [], 0, etc)
-Change: Apply the same changes as for adding to modifying.
Merge pull request #30 from Xaroth/master

Move get_db_prep_save to before value checking.