Corrected typo of variable in response.php for render method.

Added header in the category no found conditional for 404.
Added correct headers when the not_found.tpl is triggered for catalog area; download, products, category, order, returns, cart, information, manufacturers.
Removed the 404 header when the cart is empty.
Merge branch 'v1.5.6.1' of git://github.com/opencart/opencart.git into v1.5.6.1