fix some bugs, but probably not the pesky ones

- enqueuing system messages to DeadLetterMailbox was broken in
  principle, but not in practice for the current default deadLetter
  implementation
- add assert that system messages are not enqueued multiple times
- *BusSpec was using incorrect compareSubscribers based on
  identityHashCode, so moved the proper solution out of
  BalancingDispatcher and into akka.util.Helpers and reuse that in all
  places

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -222,8 +222,9 @@ abstract class MessageDispatcher(val app: ActorSystem) extends Serializable {
     if (mailBox.hasSystemMessages) {
       var message = mailBox.systemDrain()
       while (message ne null) {
+        val next = message.next
         deadLetterMailbox.systemEnqueue(message)
-        message = message.next
+        message = next
       }
     }