Fixed MediaStreamRecorder audio bug

In MediaStreamRecorder there is a (not so) dirty hack to record audio when the stream contains both video and audio (otherwise video/webm blobs are recorded).
The hack uses this.type which is not yet set in the constructor of MediaRecorderWrapper (the mergeProps is only done after construction).
As a workaround I passed the type to the constructor since the type can be used right away to change the mediastream.