Fixed #8214 -- Added back a stringification that was dropped during [8131].

This restores filtering on dates in the admin, for example. Thanks to akfarrell
and a small team of workers at the Portland Sprint.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8494 bcc190cf-cafb-0310-a4f2-bffc1f526a37