[1.2.X] Fixed #13686 -- Ensure that memcache handling of unicode values in...

[1.2.X] 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.

Backport of r15880 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15881 bcc190cf-cafb-0310-a4f2-bffc1f526a37