Reset payment details before forwarding to gateway.

### Problem

I have a payment gateway require me to set a unique "merchant reference number" to each payment, and  this information is set in `CaptureOrderAction` via `$payment->setDetails(['ref' => 'xxx'])`.

However, if the payment's details is already set, `CaptureOrderAction` will [reuse all the existing details](https://github.com/Sylius/Sylius/blob/1f76020d41f7f5dd1e96a59eb88fe6756d58dbd3/src/Sylius/Bundle/PayumBundle/Payum/Be2bill/Action/CaptureOrderUsingBe2billFormAction.php#L61-L72), and it will end up reusing the last "merchant reference number".

### Solution 

This PR will reset the payment detail before forwarding the customer to payment gateway, make sure `CaptureOrderAction` will generate a new number for that payment.