`Payment.refund()` does not refund

[radekholy24]

Hello,

Expected behavior

If you need to refund a payment, you can do this by calling the refund() method on your Payment instance:

from payments import get_payment_model
Payment = get_payment_model()
payment = Payment.objects.get()
payment.refund()

By default, the total amount would be refunded.

-- https://django-payments.readthedocs.io/en/latest/refund.html

Actual behavior

>>> from payments import get_payment_model
>>> Payment = get_payment_model()
>>> payment = Payment.objects.get()
>>> payment.refund()
>>> payment.captured_amount
Decimal("2.00")
>>> payment.status
'confirmed'

(Neither the captured_amount nor the status changes after a call to refund())

[mariofix]

Related to #399

[radekholy24]

Here is my PR to fix the issue: #410