http_date: produce leading zero on day component

tests/standalone/io/http_headers_test.dart

Index: tests/standalone/io/http_headers_test.dart
diff --git a/tests/standalone/io/http_headers_test.dart b/tests/standalone/io/http_headers_test.dart
index edaa64af38c78db4d6574f17558c0705616d8886..5442ae8bb7b7d41a360f4aa5f85cfee9d53b27cf 100644
--- a/tests/standalone/io/http_headers_test.dart
+++ b/tests/standalone/io/http_headers_test.dart
@@ -369,32 +369,32 @@ void testCookie() {
     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"