=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-actor-tests/src/test/scala/akka/routing/ConfiguredLocalRoutingSpec.scala

@@ -71,7 +71,7 @@ object ConfiguredLocalRoutingSpec {
 
   class EchoProps extends Actor {
     def receive = {
-      case "get" ⇒ sender ! context.props
+      case "get" ⇒ sender() ! context.props
     }
   }