=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-actor-tests/src/test/scala/akka/dispatch/MailboxConfigSpec.scala

@@ -260,7 +260,7 @@ class SingleConsumerOnlyMailboxVerificationSpec extends AkkaSpec(SingleConsumerO
             def receive = {
               case Ping ⇒
                 n -= 1
-                sender ! Ping
+                sender() ! Ping
                 if (n == 0)
                   context stop self
             }