Change the copy command Content-Type header to x-tar

Addresses #6107

Signed-off-by: Srini Brahmaroutu <srbrahma@us.ibm.com>

api/server/server.go

@@ -1014,6 +1014,7 @@ func postContainersCopy(eng *engine.Engine, version version.Version, w http.Resp
 
 	job := eng.Job("container_copy", vars["name"], copyData.Get("Resource"))
 	job.Stdout.Add(w)
+	w.Header().Set("Content-Type", "application/x-tar")
 	if err := job.Run(); err != nil {
 		log.Errorf("%s", err.Error())
 		if strings.Contains(err.Error(), "No such container") {

docs/sources/reference/api/docker_remote_api_v1.15.md

@@ -710,7 +710,7 @@ Copy files or folders of container `id`
 **Example response**:
 
         HTTP/1.1 200 OK
-        Content-Type: application/octet-stream
+        Content-Type: application/x-tar
 
         {{ TAR STREAM }}