http_date: produce leading zero on day component

tests/standalone/io/http_headers_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.
 
 library dart.io;
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:collection";
 import "dart:convert";
@@ skipping 351 matching lines @@
       var c = new _Cookie.fromSetCookieValue(s);
       checkCookiesEquals(cookie, c);
     }
 
     Cookie cookie;
     cookie = new Cookie(name, value);
     Expect.equals("$name=$value", cookie.toString());
     DateTime date = new DateTime.utc(2014, DateTime.JANUARY, 5, 23, 59, 59, 0);
     cookie.expires = date;
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT");
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT");
     cookie.maxAge = 567;
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT"
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT"
                 "; Max-Age=567");
     cookie.domain = "example.com";
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT"
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT"
                 "; Max-Age=567"
                 "; Domain=example.com");
     cookie.path = "/xxx";
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT"
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT"
                 "; Max-Age=567"
                 "; Domain=example.com"
                 "; Path=/xxx");
     cookie.secure = true;
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT"
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT"
                 "; Max-Age=567"
                 "; Domain=example.com"
                 "; Path=/xxx"
                 "; Secure");
     cookie.httpOnly = true;
     checkCookie(cookie, "$name=$value"
-                "; Expires=Sun, 5 Jan 2014 23:59:59 GMT"
+                "; Expires=Sun, 05 Jan 2014 23:59:59 GMT"
                 "; Max-Age=567"
                 "; Domain=example.com"
                 "; Path=/xxx"
                 "; Secure"
                 "; HttpOnly");
     cookie.expires = null;
     checkCookie(cookie, "$name=$value"
                 "; Max-Age=567"
                 "; Domain=example.com"
                 "; Path=/xxx"
@@ skipping 60 matching lines @@
   testIfModifiedSince();
   testHost();
   testEnumeration();
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
 }