http_date: produce leading zero on day component

tests/standalone/io/http_date_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 import "dart:async";
 import "dart:math";
 import "dart:io";
 
 void testParseHttpDate() {
@@ skipping 25 matching lines @@
        String expectedFormatted) {
     DateTime date;
     String formatted;
     date = new DateTime.utc(year, month, day, hours, minutes, seconds, 0);
     formatted = HttpDate.format(date);
     Expect.equals(expectedFormatted, formatted);
     Expect.equals(date, HttpDate.parse(formatted));
   }
 
   test(1999, DateTime.JUNE, 11, 18, 46, 53, "Fri, 11 Jun 1999 18:46:53 GMT");
-  test(1970, DateTime.JANUARY, 1, 0, 0, 0, "Thu, 1 Jan 1970 00:00:00 GMT");
-  test(1970, DateTime.JANUARY, 1, 9, 9, 9, "Thu, 1 Jan 1970 09:09:09 GMT");
-  test(2012, DateTime.MARCH, 5, 23, 59, 59, "Mon, 5 Mar 2012 23:59:59 GMT");
+  test(1970, DateTime.JANUARY, 1, 0, 0, 0, "Thu, 01 Jan 1970 00:00:00 GMT");
+  test(1970, DateTime.JANUARY, 1, 9, 9, 9, "Thu, 01 Jan 1970 09:09:09 GMT");
+  test(2012, DateTime.MARCH, 5, 23, 59, 59, "Mon, 05 Mar 2012 23:59:59 GMT");
 }
 
 void testParseHttpDateFailures() {
   Expect.throws(() {
     HttpDate.parse("");
   });
   String valid = "Mon, 5 Mar 2012 23:59:59 GMT";
   for (int i = 1; i < valid.length - 1; i++) {
     String tmp = valid.substring(0, i);
     Expect.throws(() {
@@ skipping 18 matching lines @@
   Expect.throws(() {
     HttpDate.parse("$valid ");
   });
 }
 
 void main() {
   testParseHttpDate();
   testFormatParseHttpDate();
   testParseHttpDateFailures();
 }