Clean up HttpHeaders to always have them in a consistent state.

tests/standalone/io/http_10_test.dart

 // (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";
@@ skipping 110 matching lines @@
 // server sets a content length so the persistent connection can be
 // used.
 void testHttp10KeepAlive() {
   HttpServer.bind("127.0.0.1", 0).then((server) {
     server.listen(
         (HttpRequest request) {
           Expect.isNull(request.headers.value('content-length'));
           Expect.equals(-1, request.contentLength);
           var response = request.response;
           response.contentLength = 1;
+          response.persistentConnection = true;
           Expect.equals("1.0", request.protocolVersion);
           response.write("Z");
           response.close();
         },
         onError: (e, trace) {
           String msg = "Unexpected error $e";
           if (trace != null) msg += "\nStackTrace: $trace";
           Expect.fail(msg);
         });
 
@@ skipping 79 matching lines @@
   });
 }
 
 
 void main() {
   testHttp10NoKeepAlive();
   testHttp10ServerClose();
   testHttp10KeepAlive();
   testHttp10KeepAliveServerCloses();
 }