Field is username not userName
Field is username not userName

With the underscore, camelize will convert `user_name` to `userName`.
When Ravelin have `username` as the field, see api: https://developer.ravelin.com/#post-/v2/customer

Error returned with using `user_name` as per gem class accessor :

    Ravelin::InvalidRequestError: 400 - JSON validation error - userName: Additional property userName is not allowed; Must validate all the schemas (allOf)

deliveroo/ravelin-ruby,deliveroo/ravelin-ruby