fixed dub playOnTrack loop forever (#1247)

lib/tasks/dub.js

@@ -83,7 +83,8 @@ class TaskDub extends TtsTask {
       action: 'playOnTrack',
       track: this.track,
       play: this.play,
-      loop: this.loop ? 'loop' : 'once',
+      // drachtio-fsmrf will convert loop from boolean to 'loop' or 'once'
+      loop: this.loop,
       gain: this.gain
     });
   }