serve http: add Last-Modified headers to files and directories

This means that using `rclone serve http` preserves modification times
when used with the http backend.

Fixes #4201

cmd/serve/http/http.go

@@ -138,6 +138,9 @@ func (s *server) serveDir(w http.ResponseWriter, r *http.Request, dirRemote stri
 	orderParm := r.URL.Query().Get("order")
 	directory.ProcessQueryParams(sortParm, orderParm)
 
+	// Set the Last-Modified header to the timestamp
+	w.Header().Set("Last-Modified", dir.ModTime().UTC().Format(http.TimeFormat))
+
 	directory.Serve(w, r)
 }
 
@@ -175,6 +178,9 @@ func (s *server) serveFile(w http.ResponseWriter, r *http.Request, remote string
 		w.Header().Set("Content-Type", mimeType)
 	}
 
+	// Set the Last-Modified header to the timestamp
+	w.Header().Set("Last-Modified", file.ModTime().UTC().Format(http.TimeFormat))
+
 	// If HEAD no need to read the object since we have set the headers
 	if r.Method == "HEAD" {
 		return