=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-docs/rst/scala/code/docs/testkit/TestKitUsageSpec.scala

@@ -117,7 +117,7 @@ object TestKitUsageSpec {
    */
   class EchoActor extends Actor {
     def receive = {
-      case msg => sender ! msg
+      case msg => sender() ! msg
     }
   }