=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/PlainWordSpec.scala

@@ -16,7 +16,7 @@ import akka.testkit.ImplicitSender
 object MySpec {
   class EchoActor extends Actor {
     def receive = {
-      case x => sender ! x
+      case x => sender() ! x
     }
   }
 }