Fix playback of representations containing both audio and video tracks

This fixes playback of muxed representations, where there are
ContentComponent elements for both audio and video. In these
cases, the initializeMediaSource method in Stream first
initializes a stream of type "video" containing both audio
and video, and the "audio" media type isn't initialized
(with a log message "No buffer was created, skipping audio data.").

Therefore, primarily set the media type to "video", if the adaptation set contains a video track.

This fixes playback of http://albin.abo.fi/~mstorsjo/dash/ondemand-combined.mpd (which worked fine in the 1.2.0 release).

A feedback agreement has been posted at https://groups.google.com/d/msg/dashjs/GW2oc8wTG8E/cC6osFFZrDoJ.