HStoreSerializer breaks nullbooleanfield

  File "/home/max/git/upgrades/env/lib/python2.7/site-packages/rest_framework/fields.py", line 643, in **init**
    assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
AssertionError: `allow_null` is not a valid option.

solution:
changed line 39 in serializers.py
from:
if model_field.null:
to:
if model_field.null and not isinstance(model_field, models.NullBooleanField):
