fixed bug with auth.impersonate(0)

VERSION

@@ -1 +1 @@
-Version 2.1.1 (2012-10-16 06:15:40) dev
+Version 2.1.1 (2012-10-16 06:19:35) dev

gluon/tools.py

@@ -2750,6 +2750,7 @@ class Auth(object):
             session.clear()
             session.update(cPickle.loads(auth.impersonator))
             self.user = session.auth.user
+            return None
         if requested_id is DEFAULT and not request.post_vars:
             return SQLFORM.factory(Field('user_id', 'integer'))
         return SQLFORM(table_user, user.id, readonly=True)