Multiple cart checkout

I'm working on a site where a user can potentially have multiple shopping carts at any given instance (same basic concept as how etsy does it). I have everything working in a way described in #1350342: Multiple cart instances. The one problem is that the Checkout button for each cart only returns the most recent cart, not the cart for the cart form it's attached to. There's a redirect in the submit handler that looks as follows:

    $form_state['redirect'] = 'checkout';

So this will also take you to the checkout page without specifying the cart. The attached patch simply adds the order_id to the redirect.