Fixed #13686 -- Ensure that memcache handling of unicode values in add() and set_many() is consistent with the handling provided by get() and set(). Thanks to nedbatchelder for the report, and to jbalogh, accuser and Jacob Burch for their work ont the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15880 bcc190cf-cafb-0310-a4f2-bffc1f526a37