[1.0.X] The database cache wasn't correctly handling expired keys. Fixed now.

The cache tests have been failing for a long time with the db backend. This
change makes them pass again, so no test changes required here.

Backport of r9942 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9943 bcc190cf-cafb-0310-a4f2-bffc1f526a37