Revise on account registration Contact update rule

[bennylp]

Some firewall leaves rport parameter with wrong port number, causing us to update our registration Contact with wrong URI.

For example, when we're behind NAT and we're registering to server in public Internet:

• our original Via header in the request:

Via: SIP/2.0/UDP 10.0.0.1;rport;branch=xyz

• Via value in the response, after the server's response passes through the firewall:

Via: SIP/2.0/UDP 10.0.0.1;rport=56284;branch=xyz

Probably we don't need to do registration update when only the port number changes.