use _sheet_name to advoid export_workbook sheet_name error

lib/tallty_import_export/export.rb

@@ -69,9 +69,10 @@ module TalltyImportExport
       title3 = workbook.styles.add_style(alignment: alignment.merge(wrap_text: true), border: border, sz: 14)
       headers = export_headers
 
-      sheet_name = respond_to?(:sheet_name) ? sheet_name : nil
+      binding.pry
+      _sheet_name = respond_to?(:sheet_name) ? self.sheet_name : nil
 
-      workbook.add_worksheet(name: sheet_name) do |sheet|
+      workbook.add_worksheet(name: _sheet_name) do |sheet|
         if respond_to?(:first_header)
           row_index = Axlsx.col_ref(headers.size - 1)
           sheet.merge_cells("A1:#{row_index}1")