default value for offerToReceiveAudio

http://w3c.github.io/webrtc-pc/#dictionary-rtcofferoptions-members
doesn't give any default values for offerToReceiveAudio or -Video.

I assume this defaults to 1 if there is a track of that kind attached 
(or more that one)  and 0 otherwise.
That way, datachannels would end up with 0 and at least one audio/video 
m-line is sendrecv.
There are some cases like two local microphones or possibly simulcast 
where I am not sure if the answer "1" is correct or obvious, so some 
spec-guidance would be helpful.