Coupon codes are ignored when paying with PayPal

[Dethon]

As title says, If you try to use a coupon code in the payment selection page and choose PayPal as your payment method, the coupon is ignored.

This happens because the "next step" button you click when choosing PayPal is not actually a submit button, so the javascript code tasked with coupon application is not triggered. The easy way to fix it is adding some coupon managing code before calling checkout.paypal.initAuthFlow(); in _payment.html.erb

I already have a patch working, so I'll submit a PR after doing some more testing