protected Object[] getOnTableRenderArgs() {
        return new Object[]{document};
    }
Gets and caches the list of UIColumns that are exportable="true", visible="true", and rendered="true".
Orders them by displayPriority so they match the UI display of the columns.

@param table the Table with columns to export
@return the List<UIColumn> that are exportable
primefaces/src/main/java/org/primefaces/component/export/TableExporter.java