Allow to send REGISTER request without expires

[kkozlik]

This PR adding ability to send REGISTERs without expires value and let the server side decide what the expires is.

This is done by setting register_expires to zero in UA config.

[jmillan]

For readability:

Suggested change

	if (this._expires) contactValue = `${this._contact};expires=${this._expires}${this._extraContactParams}`;
	extraHeaders.push(`Contact: ${contactValue}`);
	if (this._expires) extraHeaders.push(`Expires: ${this._expires}`);
	let contactValue;
	if (this._expires)
	{
	contactValue = `${this._contact};expires=${this._expires}${this._extraContactParams}`;
	extraHeaders.push(`Expires: ${this._expires}`);
	}
	else
	{
	contactValue = `${this._contact}${this._extraContactParams}`;
	}
	extraHeaders.push(`Contact: ${contactValue}`);