Fixed #14920 -- Fixed some test failures caused by caching contenttypes that...

Fixed #14920 -- Fixed some test failures caused by caching contenttypes that were loaded during a contenttype fixture test. Thanks to Karen for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14985 bcc190cf-cafb-0310-a4f2-bffc1f526a37