=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/io/EchoServer.scala

@@ -63,8 +63,8 @@ class EchoManager(handlerClass: Class[_]) extends Actor with ActorLogging {
     //#echo-manager
     case Connected(remote, local) =>
       log.info("received connection from {}", remote)
-      val handler = context.actorOf(Props(handlerClass, sender, remote))
-      sender ! Register(handler, keepOpenOnPeerClosed = true)
+      val handler = context.actorOf(Props(handlerClass, sender(), remote))
+      sender() ! Register(handler, keepOpenOnPeerClosed = true)
     //#echo-manager
   }
 

akka-docs/rst/scala/code/docs/io/IODocSpec.scala

@@ -46,7 +46,7 @@ class Server extends Actor {
       context.parent ! c
       //#server
       val handler = context.actorOf(Props[SimplisticHandler])
-      val connection = sender
+      val connection = sender()
       connection ! Register(handler)
   }
 
@@ -57,7 +57,7 @@ class Server extends Actor {
 class SimplisticHandler extends Actor {
   import Tcp._
   def receive = {
-    case Received(data) => sender ! Write(data)
+    case Received(data) => sender() ! Write(data)
     case PeerClosed     => context stop self
   }
 }
@@ -83,7 +83,7 @@ class Client(remote: InetSocketAddress, listener: ActorRef) extends Actor {
 
     case c @ Connected(remote, local) =>
       listener ! c
-      val connection = sender
+      val connection = sender()
       connection ! Register(self)
       context become {
         case data: ByteString =>

akka-docs/rst/scala/code/docs/io/UdpDocSpec.scala

@@ -22,9 +22,9 @@ object ScalaUdpDocSpec {
 
     def receive = {
       case Udp.SimpleSenderReady =>
-        context.become(ready(sender))
+        context.become(ready(sender()))
         //#sender
-        sender ! Udp.Send(ByteString("hello"), remote)
+        sender() ! Udp.Send(ByteString("hello"), remote)
       //#sender
     }
 
@@ -48,7 +48,7 @@ object ScalaUdpDocSpec {
         //#listener
         nextActor forward local
         //#listener
-        context.become(ready(sender))
+        context.become(ready(sender()))
     }
 
     def ready(socket: ActorRef): Receive = {
@@ -72,9 +72,9 @@ object ScalaUdpDocSpec {
 
     def receive = {
       case UdpConnected.Connected =>
-        context.become(ready(sender))
+        context.become(ready(sender()))
         //#connected
-        sender ! UdpConnected.Send(ByteString("hello"))
+        sender() ! UdpConnected.Send(ByteString("hello"))
       //#connected
     }