fix: 导出对于string数据，默认format是string

lib/tallty_import_export/export_form.rb

@@ -88,7 +88,7 @@ module TalltyImportExport
                 else
                   row << nil
                 end
-                formats.push(header_obj.flatten_value[col_index].format&.to_sym || (row.is_a?(String) ? :string : nil))
+                formats.push(header_obj.flatten_value[col_index].format&.to_sym || (value.value.is_a?(String) ? :string : nil))
               end
               sheet.add_row(row, style: title3, height: @row_height, types: formats)
               formats = []