Update Header.php

run in hiphop-php 
pHipHop Fatal error: Uncaught exception 'ErrorException' with message 'Argument 2 passed to ArrayObject::__construct() must be an instance of int, null given' in /var/www/system/classes/Kohana/HTTP/Header.php:299\nStack trace:\n#0 (): Kohana_Core::error_handler()
#1 /var/www/system/classes/Kohana/HTTP/Header.php(299): ArrayObject->__construct()
#2 /var/www/system/classes/Kohana/Request.php(659): Kohana_HTTP_Header->__construct()
#3 /var/www/system/classes/Kohana/Request.php(160): Kohana_Request->__construct()
#4 /var/www/index.php(117): Kohana_Request::factory()
#5 {main}