Admin querystring __isnull=False filter doesn't work

[marcob]

If I use isnull=False with an admin querystring filter I always get a IS NULL query.
Try with:

   http://127.0.0.1:8000/admin/auth/user/?email__isnull=False

   http://127.0.0.1:8000/admin/auth/user/?email__isnull=True

You'll get same results.
I fixed with a tiny patch (attached):

            elif key.endswith('__isnull'):
                value = False if value.lower() == 'false' else Tru