=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/Producers.scala

@@ -120,7 +120,7 @@ object Producers {
       def receive = {
         case msg =>
           val template = CamelExtension(context.system).template
-          sender ! template.requestBody("direct:news", msg)
+          sender() ! template.requestBody("direct:news", msg)
       }
     }
     //#RequestProducerTemplate