Delayed Offer is Supported

Delayed offer is supported since WCS3 client-server build 425-527. The delayed offer means we receive INVITE without SDP.
Once we receive such INVITE without SDP, we need to send OK with SDP and then SIP server should answer with ACK and SDP.

We do not pass such non-sdp offers through WebRTC API to avoid possible bugs. Instead of this, we consider INVITE without SDP like we create new offer and then we consider received ACK with SDP as an answer.