def to_xml_string str=''
      add_autofilter_defined_name_to_workbook
      auto_filter.apply if auto_filter.range
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << worksheet_node
      serializable_parts.each do |item|
        item.to_xml_string(str) if item
      end
      str << '</worksheet>'
    end
Serializes the worksheet object to an xml string
This intentionally does not use nokogiri for performance reasons
@return [String]