sip_session: fix for AttributeError: 'ChatManager' object has no attribute 'call'

darcs-hash:20090330100412-b2873-45202fb1f90a6c2173b8e5297f470d2900d6bef8
Fixed bug in Invitation.respond_to_reinvite() with negative reply

darcs-hash:20090330100943-58103-7b3576d824a4f2d4aa57a90628cac03fc949c27d