Update mollie_controller.rb to set order_completed to true

[Kulgar]

I was missing another piece of code from the mollie gateway gem. Even though I did some edit in spree views, I'm still using there order_just_completed? method to display a thank you message. This method needs flash['order_completed'] to be true and is normally set in the checkout update action:

https://github.com/spree/spree/blob/master/frontend/app/controllers/spree/checkout_controller.rb#L43

So, I'm guessing, setting it here to true would be a good thing to do.

Wdyt?

[Oldharlem]

The mollie checkout flow does omit the update action after a completed payment. The update action does not suite the current payment flow so this solution should restore the missing functionality. Thanks for the PR 🚀

[Kulgar]

Thank you for the merge ^^