Checkout PurchaseRequest never sends customer information, even when set

[garak]

See https://github.com/thephpleague/omnipay-stripe/blame/master/src/Message/Checkout/PurchaseRequest.php#L149

The data retrieved never includes the customer key, even if the customer reference is set (i.e. the customerReference key is present in the parameters).

The fix is pretty simple:

<?php

    public function getData()
    {
        $data = [
            'success_url' => $this->getSuccessUrl(),
            'cancel_url' => $this->getCancelUrl(),
            'payment_method_types' => $this->getPaymentMethodTypes(),
            'mode' => $this->getMode(),
            'line_items' => $this->getLineItems(),
            'customer' => $this->getCustomerReference() ?? null,  // <---- this new line is needed
        ];

        return $data;
    }