Need to define fields for billing address

[steveatkin]

5.2 BillingAddress
https://www.w3.org/TR/payment-method-basic-card/#billingaddress

At the moment the specification does not define the fields for the billing address so it is unclear if there are any internationalization issues. We will need to come back and take a look at this once it is defined.

[steveatkin]

This has now been resolved in the latest editor's draft.
https://w3c.github.io/webpayments-methods-card/#basiccardresponse

The billing address is now defined as type PaymentAddress and is covered by the PaymentAddress interface in https://w3c.github.io/browser-payment-api/#paymentaddress-interface