=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/serialization/SerializeSpec.scala

@@ -83,7 +83,7 @@ object SerializationTests {
 
   class FooActor extends Actor {
     def receive = {
-      case s: String ⇒ sender ! s
+      case s: String ⇒ sender() ! s
     }
   }
 
@@ -93,7 +93,7 @@ object SerializationTests {
 
   class NonSerializableActor(system: ActorSystem) extends Actor {
     def receive = {
-      case s: String ⇒ sender ! s
+      case s: String ⇒ sender() ! s
     }
   }