Fixed #8314 -- Fixed an infinite loop caused when submitting a session key (via

      a cookie) with no corresponding entry in the database.

This only affected the database backend, but I've applied the same fix to all
three backends for robustness.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8351 bcc190cf-cafb-0310-a4f2-bffc1f526a37