Fixed #12057 -- Corrected regression of caching performance when a model...

Fixed #12057 -- Corrected regression of caching performance when a model contained a callable default. Thanks to Michael Thornhill for the excellent assistance tracking this problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11681 bcc190cf-cafb-0310-a4f2-bffc1f526a37