PushSubscription.toJSON was the wrong return type

[FabioPinheiro]

The method toJSON on PushSubscription return a Object not a String

https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription/toJSON

https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/experimental/push/PushManager.scala#L101

The error:

scala.scalajs.runtime.UndefinedBehaviorError: An undefined behavior was detected: [object Object] is not an instance of java.lang.String
...
Caused by: java.lang.ClassCastException: [object Object] is not an instance of java.lang.String

[FabioPinheiro]

Can anyone converme this?
What is the ritgh return type? js.Any ?
How can I test this?