code: public Invoice refundInvoice(final String invoiceId, final InvoiceRefund refundOptions) {
          return doPOST(Invoices.INVOICES_RESOURCE + "/" + invoiceId + "/refund", refundOptions, Invoice.class);
      }
docs: Refund an invoice given some options
  <p/>
  Returns the refunded invoice
  
  @param invoiceId The id of the invoice to refund
  @param refundOptions The options for the refund
  @return the refunded invoice