refactor(WebhookClient): make token property non-enumerable (#2861)

* make WebhookClient#token non-enumerable

parity with Client to add the barest protection against accidental exposure

* requested change

src/client/WebhookClient.js

@@ -20,7 +20,7 @@ class WebhookClient extends BaseClient {
     super(options);
     Object.defineProperty(this, 'client', { value: this });
     this.id = id;
-    this.token = token;
+    Object.defineProperty(this, 'token', { value: token, writable: true, configurable: true });
   }
 }