Bugfix: Creating Checkout session with Coupon (#1163)

[io238]

This PR fixes issue #1163.

Description:

It's not possible to generate a Stripe Checkout session through Cashier when using Coupons via the ->withCoupon() method.

There are two issues:

(1) The Stripe API does not accept both allow_promotion_codes and discounts parameters. Stripe throws an exception even when the allow_promotion_codes parameter is set to false.

(2) The discount parameter in the session creation payload needs to be an indexed array.

Both issues seem to be in inconsistent implementation of the Checkout API. It's not very well documented and this different from the way how regular subscriptions are created (without Checkout). See https://stripe.com/docs/billing/subscriptions/discounts for more details.

Changes:

• Empty values such as 'allow_promotion_codes' = false are filtered from the payload before sending it to Stripe
• coupon nested one level deeper in the discounts parameter

[driesvints]

Thanks @io238. I've took an alternative approach for this here: #1165