django/django — pull request #3297 by jdufresne (2014-10-01 21:47:46+00:00)
Title: Fixed #23580 -- Changed NullBooleanField to handle required arguemnt.

Changed NullBooleanField to handle the required argument. When
required is True, values True and False validate while None is
invalid. To keep existing behavior with older version of Django,
NullBooleanField has required=False by default.