sip_decline release callSession if ws requestor is used (#1182)

lib/tasks/sip_decline.js

@@ -18,6 +18,11 @@ class TaskSipDecline extends Task {
     super.exec(cs);
     res.send(this.data.status, this.data.reason, {
       headers: this.headers
+    }, (err) => {
+      if (!err) {
+        // Call was successfully declined
+        cs._callReleased();
+      }
     });
     cs.emit('callStatusChange', {
       callStatus: CallStatus.Failed,