Faster cart render

I found that by passing the `line_items` collection directly to the `render` method in the orders#edit (cart) form we see significant performance gains because the partial is only rendered once. In our case we saw performance gains of view rendering of around 50% faster when about 10 items are in the cart. The gains are probably smaller in other apps (we have overriding the partial using Deface) but still worthwhile. 

I think this could be useful for master branch as well as 2-0-stable but I haven't tried it yet (not on rails 4 yet).