fix gather should not play audio if gather already resolved (#638)

lib/tasks/gather.js

@@ -845,6 +845,9 @@ class TaskGather extends SttTask {
     if (this.resolved) return;
 
     this.resolved = true;
+    // If bargin is false and ws application return ack to verb:hook
+    // the gather should not play any audio
+    this._killAudio(this.cs);
     // Clear dtmf event
     if (this.dtmfBargein) {
       this.ep.removeAllListeners('dtmf');