message: fix mediaSource overwriting for Firefox screen constraint
diff: @@ -85,7 +85,9 @@ const GetUserMedia = (config, callback = () => {}, error = () => {}) => {
  screenConfig = {};
  if (config.video !== undefined) {
  screenConfig.video = config.video;
  + if (!screenConfig.video.mediaSource) {
  screenConfig.video.mediaSource = 'window' || 'screen';
  + }
  } else {
  screenConfig = {
  audio: config.audio,
  