session2: proper reaction to InvitationChangedState

darcs-hash:20090423091448-b2873-a4339dd0668e919e5cd6f67f28afa46c31eadccf
session2: allow _terminate()'s param be None

darcs-hash:20090423091521-b2873-83f05e9844dd032ef8952202d5a6d89aa56af6b6