+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

akka-http-tests/src/test/scala/akka/http/scaladsl/marshalling/MarshallingSpec.scala

@@ -27,6 +27,9 @@ class MarshallingSpec extends FreeSpec with Matchers with BeforeAndAfterAll with
     "StringMarshaller should marshal strings to `text/plain` content in UTF-8" in {
       marshal("Ha“llo") shouldEqual HttpEntity("Ha“llo")
     }
+    "DoneMarshaller should enable marshalling of akka.Done" in {
+      marshal(akka.Done) shouldEqual HttpEntity("")
+    }
     "CharArrayMarshaller should marshal char arrays to `text/plain` content in UTF-8" in {
       marshal("Ha“llo".toCharArray) shouldEqual HttpEntity("Ha“llo")
     }

akka-http/src/main/scala/akka/http/scaladsl/marshalling/PredefinedToEntityMarshallers.scala

@@ -7,6 +7,7 @@ package akka.http.scaladsl.marshalling
 import java.nio.CharBuffer
 
 import akka.http.scaladsl.model.MediaTypes._
+import akka.http.scaladsl.model.ContentTypes.`text/plain(UTF-8)`
 import akka.http.scaladsl.model._
 import akka.util.ByteString
 
@@ -35,6 +36,11 @@ trait PredefinedToEntityMarshallers extends MultipartMarshallers {
       HttpEntity(contentType, array)
     } else HttpEntity.Empty
 
+  implicit val DoneMarshaller: ToEntityMarshaller[akka.Done] =
+    Marshaller.withFixedContentType(`text/plain(UTF-8)`) { done ⇒
+      HttpEntity(`text/plain(UTF-8)`, "")
+    }
+
   implicit val StringMarshaller: ToEntityMarshaller[String] = stringMarshaller(`text/plain`)
   def stringMarshaller(mediaType: MediaType.WithOpenCharset): ToEntityMarshaller[String] =
     Marshaller.withOpenCharset(mediaType) { (s, cs) ⇒ HttpEntity(mediaType withCharset cs, s) }

akka-http/src/main/scala/akka/http/scaladsl/marshalling/PredefinedToResponseMarshallers.scala

@@ -4,6 +4,8 @@
 
 package akka.http.scaladsl.marshalling
 
+import akka.stream.impl.ConstantFun
+
 import scala.collection.immutable
 import akka.http.scaladsl.util.FastFuture._
 import akka.http.scaladsl.model.MediaTypes._
@@ -18,7 +20,7 @@ trait PredefinedToResponseMarshallers extends LowPriorityToResponseMarshallerImp
                                   implicit m: ToEntityMarshaller[T]): ToResponseMarshaller[T] =
     fromStatusCodeAndHeadersAndValue compose (t ⇒ (status, headers, t))
 
-  implicit val fromResponse: TRM[HttpResponse] = Marshaller.opaque(conforms)
+  implicit val fromResponse: TRM[HttpResponse] = Marshaller.opaque(ConstantFun.scalaIdentityFunction)
 
   implicit val fromStatusCode: TRM[StatusCode] =
     Marshaller.withOpenCharset(`text/plain`) { (status, charset) ⇒