Fixed #15264 -- Ensured that legacy cache backends specified using the legacy...

Fixed #15264 -- Ensured that legacy cache backends specified using the legacy cache format continue to work. Thanks to Stephane for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15488 bcc190cf-cafb-0310-a4f2-bffc1f526a37