Don't have the ModelSerializer trust deserialized objects to not have redefine bool()ean-ness.

If the model we're using the ModelSerializer for has redefined methods that act as a boolean (__bool__ or __len__), it may not return the object even though it is_valid(), and should.
Merge pull request #772 from preed/preed/ModelSerializer-bool-fix

Don't have the ModelSerializer trust deserialized objects to not have re...