Fixed the error "SIP/2.0 481 Call leg unavailable" returned by the server when

Fixed the error "SIP/2.0 481 Call leg unavailable" returned by the server when
the Routes changed in a session with already dialog. It happens with Windows Messenger 5.5 and sometimes with our own SIPE (old versions).

The server gives this reason:
ms-diagnostics: 1004;reason="Route set is no longer valid";source="myserver.org"
;ErrorType="Connection identifier encoded into the route set references a
connection instance that no longer exists"

The problem is related to the headers "Route:", I can't reproduce it, but
I think happens when:
+ John Doe send us a message
+ I give an answer and leave open the chat window
+ John Doe send us again a message after some time
+ I try to give an answer but I got the message: "This message was not delivered
 to John Doe because one or more recipients are offline".

I fixed it deleting all routes for the dialog and recording the new list.