Add payment-method-nonce to Customer

[lquach]

To create a payment method along with a customer.
The Ruby documentation: https://developers.braintreepayments.com/reference/request/customer/create/ruby#customer-with-a-payment-method

[leighmcculloch]

@lquach: Thanks! Could you add a test to customer_integration_test.go that creates a customer using one of the fake nonces?

[lquach]

Test added.

[leighmcculloch]

Nice! Thank you!

[lquach]

@leighmcculloch thank you!

[leighmcculloch]

Aside to this we should probably look at splitting Customer into Customer (the data returned) and CustomerRequest (the data sent when creating/updating) like we recently did to Transaction, since it now has PaymentMethodNonce which is only used when creating the customer and not in the response.