Fixed #13946 -- Modified the database cache backend to use the database router...

Fixed #13946 -- Modified the database cache backend to use the database router to determine availability of the cache table. Thanks to tiemonster for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13473 bcc190cf-cafb-0310-a4f2-bffc1f526a37