=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/camel/CustomRoute.scala

@@ -19,7 +19,7 @@ object CustomRoute {
     class Responder extends Actor {
       def receive = {
         case msg: CamelMessage =>
-          sender ! (msg.mapBody {
+          sender() ! (msg.mapBody {
             body: String => "received %s" format body
           })
       }
@@ -53,7 +53,7 @@ object CustomRoute {
         handled(true).transform(Builder.exceptionMessage).end
 
       final override def preRestart(reason: Throwable, message: Option[Any]) {
-        sender ! Failure(reason)
+        sender() ! Failure(reason)
       }
     }
     //#ErrorThrowingConsumer