Bug: Hangup and Refer broken

[MirzaBegunic]

While using diago library I've noticed that Hangup and Refer are currently broken. Reason why is that Content-Length property isn't set when building SIP requests for these actions. I've created forks of sipgo to fix hangup issue and diago to fix refer issue. I can create pull requests, but since is small one line fix maybe its easier for you to fix it manually in some upcoming release.

[emiago]

@MirzaBegunic Create a PR and we can discuss.