Fix Null Sender issue

[Aaronontheweb]

Per what we discussed on Gitter:

Receive<GetCurrentLifecycleStage>(stage =>
            {
                if(_currentSessionState != null)
                    Sender.Tell(_currentSessionState); //SENDER can be null!
            });

We need to make it so Sender can't be null. Solution is to do what JVM Akka does:

  /*
   * ACTOR CONTEXT IMPLEMENTATION
   */

  final def sender(): ActorRef = currentMessage match {
    case null                      ⇒ system.deadLetters
    case msg if msg.sender ne null ⇒ msg.sender
    case _                         ⇒ system.deadLetters
  }

Coalesce the null into Deadletters.