Add 'Server' and 'User-Agent' default header fields.

pkg/http/test/request_test.dart

 // Copyright (c) 2013, 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 request_test;
 
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
 import 'package:http/src/utils.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
   test('.send', () {
     expect(startServer().then((_) {
 
       var request = new http.Request('POST', serverUrl);
       request.body = "hello";
+      request.headers['User-Agent'] = 'Dart';
 
       expect(request.send().then((response) {
         expect(response.statusCode, equals(200));
         return response.stream.bytesToString();
       }).whenComplete(stopServer), completion(parse(equals({
         'method': 'POST',
         'path': '/',
         'headers': {
           'content-type': ['text/plain; charset=utf-8'],
           'accept-encoding': ['gzip'],
+          'user-agent': ['Dart'],
           'content-length': ['5']
         },
         'body': 'hello'
       }))));
     }), completes);
   });
 
   group('#contentLength', () {
     test('is computed from bodyBytes', () {
       var request = new http.Request('POST', dummyUrl);
@@ skipping 347 matching lines @@
   });
 
   group('#toString()', () {
     test('includes the method and URL', () {
       var request = new http.Request('POST', dummyUrl);
       expect(request.toString(), 'POST $dummyUrl');
     });
   });
 }