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

sdk/lib/io/http.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.
 
 part of dart.io;
 
 /**
  * HTTP status codes.
  */
 abstract class HttpStatus {
@@ skipping 930 matching lines @@
    * is not available.
    */
   HttpConnectionInfo get connectionInfo;
 }
 
 
 /**
  * HTTP response for a client connection. The [HttpClientResponse] is a
  * [Stream] of the body content of the response. Listen to the body to handle
  * the data and be notified once the entire body is received.
-
  */
 abstract class HttpClientResponse implements Stream<List<int>> {
   /**
    * Returns the status code.
    */
   int get statusCode;
 
   /**
    * Returns the reason phrase associated with the status code.
    */
@@ skipping 163 matching lines @@
 class RedirectLimitExceededException extends RedirectException {
   const RedirectLimitExceededException(List<RedirectInfo> redirects)
       : super("Redirect limit exceeded", redirects);
 }
 
 
 class RedirectLoopException extends RedirectException {
   const RedirectLoopException(List<RedirectInfo> redirects)
       : super("Redirect loop detected", redirects);
 }