Fix bugs on contact header

[suapapa]

I made an b2buas with go-sip-ua. It works well. Thanks alot! But, found some bug's when run it/them behind SIP load balancer:

• contact header contain LB's address
• 200 for BYE to wrong destination (port)

This patches are the fix I'm using.

[cloudwebrtc]

LGTM