=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/routing/ConsistentHashingRouterDocSpec.scala

@@ -19,7 +19,7 @@ object ConsistentHashingRouterDocSpec {
 
     def receive = {
       case Entry(key, value) => cache += (key -> value)
-      case Get(key)          => sender ! cache.get(key)
+      case Get(key)          => sender() ! cache.get(key)
       case Evict(key)        => cache -= key
     }
   }