code: public Invoice updateInvoice(final String invoiceId, final Invoice invoice) {
          return doPUT(Invoices.INVOICES_RESOURCE + "/" + invoiceId, invoice, Invoice.class);
      }
docs: Update an invoice
  <p>
  Updates an existing invoice.
  
  @param invoiceId String Recurly Invoice ID
  @return the updated invoice object on success, null otherwise