Fixed #13328 -- Added a __getstate__/__setstate__ pair to fields so that...

Fixed #13328 -- Added a __getstate__/__setstate__ pair to fields so that callable default values aren't pickled. Thanks to bkonkle for the report, and Vitaly Babiy for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12977 bcc190cf-cafb-0310-a4f2-bffc1f526a37