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

tests/standalone/io/http_detach_socket_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
 
 void testServerDetachSocket() {
   HttpServer.bind("127.0.0.1", 0).then((server) {
+    server.serverHeader = null;
     server.listen((request) {
       var response = request.response;
       response.contentLength = 0;
       response.detachSocket().then((socket) {
         Expect.isNotNull(socket);
         var body = new StringBuffer();
         socket.listen(
           (data) => body.write(new String.fromCharCodes(data)),
           onDone: () => Expect.equals("Some data", body.toString()));
         socket.write("Test!");
@@ skipping 56 matching lines @@
                         "content-length: 0\r\n"
                         "host: 127.0.0.1:${server.port}\r\n\r\n"
                         "Some data",
                         body.toString());
           socket.close();
         });
       server.close();
     });
 
     var client = new HttpClient();
+    client.userAgent = null;
     client.get("127.0.0.1", server.port, "/")
       .then((request) => request.close())
       .then((response) {
         response.detachSocket().then((socket) {
           var body = new StringBuffer();
           socket.listen(
             (data) => body.write(new String.fromCharCodes(data)),
             onDone: () {
               Expect.equals("Test!",
                             body.toString());
               client.close();
             });
           socket.write("Some data");
           socket.close();
         });
       });
   });
 }
 
 void main() {
   testServerDetachSocket();
   testBadServerDetachSocket();
   testClientDetachSocket();
 }