header format can be nil

lib/tallty_import_export/export.rb

@@ -84,7 +84,7 @@ module TalltyImportExport
         records.send(each_method).with_index do |record, index|
           row = []
           headers.each{ |header| row.push(handle_data(record, header, index)) }
-          sheet.add_row row, style: title3, height: @row_height, types: headers.map{|header| header[:format].to_sym}
+          sheet.add_row row, style: title3, height: @row_height, types: headers.map{|header| header[:format]&.to_sym}
         end
         sheet.column_widths(*headers.map{|header| header[:width] || @width})
       end