Give hidden field in cart form the right name so it populates order

Thanks for creating a 2-2-stable branch for this extension!

I tried to use it, but found I couldn't add a variant to the cart. Digging into it, it was because the cart form was sending "products[x] => y" to orders#populate instead of the variant_id. 

Admittedly, I'm not sure of the ramifications of the change I'm making, except that it seems to fix my problem in the limited testing I've done. :-) 

Thought you might want to include this in your 2-2-stable branch. Let me know if you need me to do anything else to make this useful.