Example UA sends SIP BYE about go-sip-ua

[oogali]

	bye, err := sip.NewRequestBuilder().
		SetMethod("BYE").
		SetContact(contact).
		SetFrom(from).
		SetTo(to).
		SetCallID(callID).
		SetUserAgent(userAgent).
		SetBody("").
		Build()
	if err != nil {
		// ...
	}

	if err := s.Send(bye); err != nil {
		// ...
	}

[chumvan]

Thank you, @oogali. I guess we need to extend the ua package to add this new feature.