=htc #20920 create ContentLength HTTP header in JavaDSL (#20926)

remove constructor

akka-http-core/src/main/java/akka/http/javadsl/model/headers/ContentLength.java

@@ -0,0 +1,13 @@
+/**
+ * Copyright (C) 2016 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package akka.http.javadsl.model.headers;
+
+/**
+ *  Model for the `Content-Length` header.
+ *  Specification: https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-26#section-3.3.2
+ */
+public abstract class ContentLength extends akka.http.scaladsl.model.HttpHeader {
+    public abstract long length();
+}

akka-http-core/src/main/scala/akka/http/scaladsl/model/headers/headers.scala

@@ -377,7 +377,8 @@ object `Content-Length` extends ModeledCompanion[`Content-Length`]
  * Instances of this class will only be created transiently during header parsing and will never appear
  * in HttpMessage.header. To access the Content-Length, see subclasses of HttpEntity.
  */
-final case class `Content-Length` private[http] (length: Long) extends RequestResponseHeader {
+final case class `Content-Length` private[http] (length: Long) extends jm.headers.ContentLength
+  with RequestResponseHeader {
   def renderValue[R <: Rendering](r: R): r.type = r ~~ length
   protected def companion = `Content-Length`
 }