We must document these APIs asap (we should really be doing this as part of writing the code anyway).

In this case I'd expect answer to be a string which is the sdp from the other end, but the tests imply an object. If its an object, then the caller needs to do new mozRTCSessionDescription(answer) on the string, which seems like something that should be encapsulated in the model to save WebRTC items leaking out.