=act #3572 Add parens to sender

* because it is not referentially transparent; normally we reserved parens for
  side-effecting code but given how people thoughtlessly close over it we revised
  that that decision for sender
* caller can still omit parens

akka-cluster/src/multi-jvm/scala/akka/cluster/MultiNodeClusterSpec.scala

@@ -68,7 +68,7 @@ object MultiNodeClusterSpec {
         }
       case End ⇒
         testActor forward End
-        sender ! EndAck
+        sender() ! EndAck
       case EndAck ⇒
         testActor forward EndAck
     }