Dash Adapter does not support Default track from stream (#219)

See here JSDoc: Class: shaka.extern
the Track.primary filed in dash parsed track (return form shaka.getTextTracks() API) indicates the default track
Which ignored by the adapter
(Unlike Hls Adapter which handles default track which indicated by Track.default field there)

solves FEC-12776

src/dash-adapter.js

@@ -1050,6 +1050,7 @@ export default class DashAdapter extends BaseMediaSourceAdapter {
         id: textTrack.id,
         kind: kind,
         active: false,
+        default: textTrack.primary,
         label: textTrack.label,
         language: textTrack.language
       };