Fix StripeProviderV3.refund() returning cents instead of currency units

[YounesOMK]

Fixes #470

[mariofix]

@YounesOMK Are we sure about this? I understand Stripe always uses the smallest amount possible for the currency

[YounesOMK]

@YounesOMK Are we sure about this? I understand Stripe always uses the smallest amount possible for the currency

@mariofix
The conversion to cents still happens for the API call, that part didn't change. the fix is only about what gets returned. Before, the refund method returned the converted amount (in cents) back to BasePayment.refund(), which does self.captured_amount -= amount. Since captured_amount is stored in currency units, the subtracted amount must also be in the same unit otherwise we will have negative values in captured_amount field