session2.NotificationHandler: poke __dict__ directly instead of attribute access

- attribute access got confused when proxies are involved

darcs-hash:20090427115620-b2873-2b1fa5745d5c17c3555141ebddd98f0f1fcbb842
session2: expect 'streams' attribute to be None

darcs-hash:20090427115826-b2873-43e308f1c04b98abb23e04ac62cdf0d6cda02eb6