fix: gather task should clear dtmf event before resolve (#284)

Co-authored-by: Quan HL <quanluuhoang8@gmail.com>

lib/tasks/gather.js

@@ -788,6 +788,10 @@ class TaskGather extends Task {
     if (this.resolved) return;
 
     this.resolved = true;
+    // Clear dtmf event
+    if (this.dtmfBargein) {
+      this.ep.removeAllListeners('dtmf');
+    }
     clearTimeout(this.interDigitTimer);
     this._clearTimer();