Add missing certificate getter on HttpClientResponse and add a regression test.

tests/standalone/io/https_client_certificate_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.
 
 import "dart:async";
 import "dart:io";
 import "dart:uri";
 import "dart:isolate";
 
 const SERVER_ADDRESS = "127.0.0.1";
@@ skipping 10 matching lines @@
     server.listen((HttpRequest request) {
       Expect.isNotNull(request.certificate);
       Expect.equals('CN=localhost', request.certificate.subject);
       request.response.addString("Hello");
       request.response.close();
     });
 
     HttpClient client = new HttpClient();
     client.getUrl(Uri.parse("https://$HOST_NAME:${server.port}/"))
         .then((request) => request.close())
-        .then((response) =>
-            response.reduce(<int>[], (message, data) => message..addAll(data)))
+        .then((response) {
+          Expect.equals('CN=localhost', response.certificate.subject);
+          Expect.equals('CN=myauthority', response.certificate.issuer);
+          return response.reduce(<int>[],
+                                 (message, data) => message..addAll(data));
+        })
         .then((message) {
           String received = new String.fromCharCodes(message);
           Expect.equals(received, "Hello");
           client.close();
           server.close();
           keepAlive.close();
         });
   });
 }
 
 void InitializeSSL() {
   var testPkcertDatabase =
       new Path(new Options().script).directoryPath.append('pkcert/');
   SecureSocket.initialize(database: testPkcertDatabase.toNativePath(),
                           password: 'dartdart');
 }
 
 void main() {
   InitializeSSL();
   test();
 }