Save correct cardholder name in Spree::CreditCard

[spaghetticode]

When paying with Payment Intents either using the HTML form or the payment request button, we need to properly detect the cardholder name in order to save the correct information in the credit card DB record.

Fixes #68

[aldesantis]

@spaghetticode the change itself looks good, but with all these methods and conditionals that extract data from Stripe's response I feel like there's a lot of indirection going on which makes it hard to figure out what's happening. At some point (or even in this PR if you have time) we may want to streamline them by moving more stuff to payment_params — it may make the method longer, but it will also make it easier to understand.

[spaghetticode]

@aldesantis yes, I was not 100% comfortable with the added logic as well, but I think ATM fixing the bug would be the priority - after all, it's private methods. I am not sure about complicating payment_params, I would like to explore also the idea of extracting all that logic into another class.

[aldesantis]

Sounds good, this is GTM as far as I'm concerned!