Make the default HTTP server configuration more secure

tests/standalone/io/http_server_response_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:async";
 import "dart:io";
 import "dart:typed_data";
 
 void testServerRequest(void handler(server, request),
                        {int bytes,
                         bool closeClient}) {
   HttpServer.bind("127.0.0.1", 0).then((server) {
+    server.defaultResponseHeaders.clear();
     server.listen((request) {
       handler(server, request);
     });
 
     var client = new HttpClient();
     // We only close the client on either
     // - Bad response headers
     // - Response done (with optional errors in between).
     client.get("127.0.0.1", server.port, "/")
       .then((request) => request.close())
@@ skipping 275 matching lines @@
   testResponseDone();
   testResponseAddStream();
   testResponseAddStreamClosed();
   testResponseAddClosed();
   testBadResponseAdd();
   testBadResponseClose();
   testIgnoreRequestData();
   testBadHeaders();
   testWriteCharCode();
 }