Now sending users to Stripe Checkout during the process method

[dparker1005]

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Changes proposed in this Pull Request:

Previously, we had to send users to Stripe Checkout on the pmpro_checkout_before_change_membership_level hook since user objects would not exist during the gateway's process() method. Now, however, users are created before payments are taken.

Currently, if sending a user to Stripe Checkout fails (due to not being able to connect to Stripe, for example), the checkout will complete successfully without collecting payment. Switching to using the process() method allows us to halt checkout on these errors and show a relevant error message at checkout.

The easiest way to replicate is to set up a local dev environment and check out with Stripe Checkout with your machine's wifi disabled.

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Changelog entry

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[dparker1005]

This should wait for a minor release in case custom code is calling our Stripe gateway's pmpro_checkout_before_change_membership_level() method. This is extremely unlikely, but technically breaking

[andrewlimaza]

This works out well, thanks David!