code: public BillingInfo createOrUpdateBillingInfo(final String accountCode, final BillingInfo billingInfo) {
          return doPUT(Account.ACCOUNT_RESOURCE + "/" + accountCode + BillingInfo.BILLING_INFO_RESOURCE,
                       billingInfo, BillingInfo.class);
      }
docs: Update an account's billing info
  <p>
  When new or updated credit card information is updated, the billing information is only saved if the credit card
  is valid. If the account has a past due invoice, the outstanding balance will be collected to validate the
  billing information.
  <p>
  If the account does not exist before the API request, the account will be created if the billing information
  is valid.
  <p>
  Please note: this API end-point may be used to import billing information without security codes (CVV).
  Recurly recommends requiring CVV from your customers when collecting new or updated billing information.
  
  @param accountCode recurly account id
  @param billingInfo billing info object to create or update
  @return the newly created or update billing info object on success, null otherwise