[Serusers] NAT contat rewrite

[Linus Surguy]

> Contacts are used to advertised wherebouts for subsequent in-dialog
> requests coming back to your UAC. For that reason, I don't entirely
> understand why the UAC includes Contact in CANCEL at all -- all in all
> CANCEL is used to discontinued estabslishment of a dialog. Anyhow,
> it should not matter the Contact content is invalid since there is no
> use for the header field in this case at all.

Maybe I've misinterpreted my fault then, the problem I've got exhibits
itself when the call is forwarded to an AS5300, the INVITE goes through fine
and the call is placed, but the CANCEL (UA->SER->AS5300) is responded to
with '481 call leg / transaction does not exist' and the call is not
canceled.

I had made the assumption that it was the contact header that was at fault
here as that was the obvious difference.

Linus