Fixed edge case that breaks the test suite on versions of Python > 2.6.4

Before http://svn.python.org/view?view=rev&revision=74647 it was possible to
pass a SimpleCookie to load, but this no longer works due to a different bug
in Python the said revision fixed.

My guess is a SimpleCookie was never intended to be passed through load which
is perfectly reasonable.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11820 bcc190cf-cafb-0310-a4f2-bffc1f526a37