Fixed #17256 -- Ensured that content types get cached when retrieved by...

Fixed #17256 -- Ensured that content types get cached when retrieved by natural key. Thanks, defaultwombat and charettes.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17502 bcc190cf-cafb-0310-a4f2-bffc1f526a37