=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-samples/akka-sample-main-scala/src/main/scala/sample/hello/Greeter.scala

@@ -14,6 +14,6 @@ class Greeter extends Actor {
   def receive = {
     case Greeter.Greet =>
       println("Hello World!")
-      sender ! Greeter.Done
+      sender() ! Greeter.Done
   }
 }