remove excessive session saving with caching of mobile_agent

VERSION

@@ -1 +1 @@
-Version 2.1.1 (2012-10-17 14:29:55) dev
+Version 2.1.1 (2012-10-17 14:33:49) dev

gluon/tools.py

@@ -1080,7 +1080,7 @@ class Auth(object):
                 auth.last_visit = request.now
         else:
             self.user = None
-            session.auth = None
+            if session.auth: del session.auth
         # ## what happens after login?
 
         self.next = current.request.vars._next