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

tests/standalone/io/https_client_certificate_test.dart

Index: tests/standalone/io/https_client_certificate_test.dart
diff --git a/tests/standalone/io/https_client_certificate_test.dart b/tests/standalone/io/https_client_certificate_test.dart
index 1d9b0e4180c6d867d85c887a716b59e2500cd44c..8a5cf7d7fae0fed27d478c2e4426fcc497e792df 100644
--- a/tests/standalone/io/https_client_certificate_test.dart
+++ b/tests/standalone/io/https_client_certificate_test.dart
@@ -28,8 +28,11 @@ Function test() {
     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));

[Søren Gjesse, 2013/03/01 12:25:48]

Long line.

[Mads Ager (google), 2013/03/01 12:31:25]

Done.

+        })
         .then((message) {
           String received = new String.fromCharCodes(message);
           Expect.equals(received, "Hello");