The database cache wasn't correctly handling expired keys. Fixed now.
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.

--HG--
extra : convert_revision : svn%3Abcc190cf-cafb-0310-a4f2-bffc1f526a37/django/trunk%409942

Belgabor/django,Belgabor/django,Belgabor/django