[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.

Sorry, just realised that in a half-awake state I typed rubbish. I meant
that the 'call-id' was not rewritten - don't know why I mentioned contact!
(In the INVITE to the Cisco it is blah at 213.x.x.x and in the CANCEL to Cisco
it is blah at 10.x.x.x

Linus