Request now has the same body model as Response

lib/src/util.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library shelf.util;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 22 matching lines @@
 ///
 /// For keys that are the same between [original] and [updates], the value in
 /// [updates] is used.
 ///
 /// If [updates] is `null` or empty, [original] is returned unchanged.
 Map updateMap(Map original, Map updates) {
   if (updates == null || updates.isEmpty) return original;
 
   return new Map.from(original)..addAll(updates);
 }
+
+/// Adds a header with [name] and [value] to [headers], which may be null.
+///
+/// Returns a new map without modifying [headers].
+Map<String, String> addHeader(
+    Map<String, String> headers, String name, String value) {
+  headers = headers == null ? {} : new Map.from(headers);
+  headers[name] = value;
+  return headers;
+}