=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/remoting/RemoteDeploymentDocSpec.scala

@@ -13,7 +13,7 @@ import akka.remote.RemoteScope
 object RemoteDeploymentDocSpec {
 
   class SampleActor extends Actor {
-    def receive = { case _ => sender ! self }
+    def receive = { case _ => sender() ! self }
   }
 
 }