Prevent exception when calling Invitation.disconnect() in the INCOMING state

darcs-hash:20090130153247-58103-9ffd7399ce8ebedd32ce912b0b158a643347f18b
Don't give the code/reason when Invitation moves into DISCONNECTED state and user sent negative final response

darcs-hash:20090130154245-58103-4090f3601ebfcd1ea814f1105f5a007579f9c3dd