+htc #19708 provides default marshaller for akka.Done (200, empty text response)

akka-docs/rst/scala/code/docs/http/scaladsl/server/directives/FutureDirectivesExamplesSpec.scala

@@ -5,6 +5,7 @@
 package docs.http.scaladsl.server.directives
 
 import java.util.concurrent.TimeUnit
+import akka.http.scaladsl.marshalling.ToResponseMarshallable
 import docs.http.scaladsl.server.RoutingSpec
 
 import scala.concurrent.Future