[1.1.X] Fixed #11483 -- Modified db cache backend to use db backend functions...

[1.1.X] Fixed #11483 -- Modified db cache backend to use db backend functions for date conversion, avoiding problems under Jython. Thanks to Leo Soto for the report and patch.

Merge of r12411 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12415 bcc190cf-cafb-0310-a4f2-bffc1f526a37