=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-remote-tests/src/multi-jvm/scala/akka/remote/routing/RemoteRoundRobinSpec.scala

@@ -29,7 +29,7 @@ object RemoteRoundRobinMultiJvmSpec extends MultiNodeConfig {
 
   class SomeActor extends Actor {
     def receive = {
-      case "hit" ⇒ sender ! self
+      case "hit" ⇒ sender() ! self
     }
   }