=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/PublishSubscribe.scala

@@ -27,7 +27,7 @@ object PublishSubscribe {
     def receive = {
       case msg: CamelMessage => {
         publisher ! msg.bodyAs[String]
-        sender ! ("message published")
+        sender() ! ("message published")
       }
     }
   }