Fixed #5981 -- Fixed failing regression test when using locmem cache backend. ...

Fixed #5981 -- Fixed failing regression test when using locmem cache backend.  Changed `add` to pickle the value as is done in `set`.  Based on patch from mattmcc.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6709 bcc190cf-cafb-0310-a4f2-bffc1f526a37