Fixed a bug in `NullBooleanField.to_python`.
Fixed a bug in `NullBooleanField.to_python`.

--HG--
extra : convert_revision : svn%3Abcc190cf-cafb-0310-a4f2-bffc1f526a37/django/trunk%408684

Belgabor/django,Belgabor/django,Belgabor/django