=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-contrib/src/multi-jvm/scala/akka/contrib/pattern/ClusterShardingSpec.scala

@@ -98,7 +98,7 @@ object ClusterShardingSpec extends MultiNodeConfig {
     override def receiveCommand: Receive = {
       case Increment      ⇒ persist(CounterChanged(+1))(updateState)
       case Decrement      ⇒ persist(CounterChanged(-1))(updateState)
-      case Get(_)         ⇒ sender ! count
+      case Get(_)         ⇒ sender() ! count
       case ReceiveTimeout ⇒ context.parent ! Passivate(stopMessage = Stop)
       case Stop           ⇒ context.stop(self)
     }