def to_xml_string(str='')
      add_worksheet(name: 'Sheet1') unless worksheets.size > 0
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<workbook xmlns="' << XML_NS << '" xmlns:r="' << XML_NS_R << '">')
      str << ('<workbookPr date1904="' << @@date1904.to_s << '"/>')
      views.to_xml_string(str)
      str << '<sheets>'
      if is_reversed
        worksheets.reverse_each { |sheet| sheet.to_sheet_node_xml_string(str) }
      else
        worksheets.each { |sheet| sheet.to_sheet_node_xml_string(str) }
      end
      str << '</sheets>'
      defined_names.to_xml_string(str)
      unless pivot_tables.empty?
        str << '<pivotCaches>'
        pivot_tables.each do |pivot_table|
          str << ('<pivotCache cacheId="' << pivot_table.cache_definition.cache_id.to_s << '" r:id="' << pivot_table.cache_definition.rId << '"/>')
        end
        str << '</pivotCaches>'
      end
      str << '</workbook>'
    end
Serialize the workbook
@param [String] str
@return [String]