code: public int getInvoicesCount(final QueryParams params) {
          FluentCaseInsensitiveStringsMap map = doHEAD(Invoices.INVOICES_RESOURCE, params);
          return Integer.parseInt(map.getFirstValue(X_RECORDS_HEADER_NAME));
      }
docs: Return all the invoices given query params
  <p>
  
  @param params {@link QueryParams}
  @return the count of invoices matching the query