Fix for message dispatcher attach (and async init)

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

@@ -72,19 +72,22 @@ abstract class MessageDispatcher extends Serializable {
   /**
    * Attaches the specified actor instance to this dispatcher
    */
-  final def attach(actor: ActorInstance) {
-    guard withGuard {
+  final def attach(actor: ActorInstance): Unit = {
+    val promise = new ActorPromise(Timeout.never)(this)
+    guard.lock.lock()
+    try {
       register(actor)
-      val promise = new ActorPromise(Timeout.never)(this)
       dispatchMessage(new MessageInvocation(actor, Init, promise))
-      promise
-    }.get
+    } finally {
+      guard.lock.unlock()
+    }
+    promise.get
   }
 
   /**
    * Detaches the specified actor instance from this dispatcher
    */
-  final def detach(actor: ActorInstance) {
+  final def detach(actor: ActorInstance): Unit = {
     guard withGuard {
       unregister(actor)
     }