Explicitly reject unsupported m= lines

CreateAnswer currently omits any m= lines from the offer that contain
unsupported codecs. The jsep spec says that unsupported lines in the
offer need to be explicitly rejected by setting the port number to 0.

Related to #449

peerconnection.go

@@ -1411,6 +1411,14 @@ func (pc *PeerConnection) addFingerprint(d *sdp.SessionDescription) {
 
 func (pc *PeerConnection) addRTPMediaSection(d *sdp.SessionDescription, codecType RTPCodecType, midValue string, iceParams ICEParameters, peerDirection RTPTransceiverDirection, candidates []ICECandidate, dtlsRole sdp.ConnectionRole) bool {
 	if codecs := pc.api.mediaEngine.getCodecsByKind(codecType); len(codecs) == 0 {
+		d.WithMedia(&sdp.MediaDescription{
+			MediaName: sdp.MediaName{
+				Media:   codecType.String(),
+				Port:    sdp.RangedPort{Value: 0},
+				Protos:  []string{"UDP", "TLS", "RTP", "SAVPF"},
+				Formats: []string{"0"},
+			},
+		})
 		return false
 	}
 	media := sdp.NewJSEPMediaDescription(codecType.String(), []string{}).