Always send the current billing name to Stripe

This didn't return the concatenated billing name for me `card_name += $(el).val() + ' ';` (Which may have must been something to do with the WooCommerce installation I was working with).

I set the first and last name more verbosely and made sure updated billing details are set as the card name hidden field value.

The code is probably a bit bloated and poor—a results of my tiny js knowledge—but hopefully the intention is good.