fix: export headers deep_symbolize_keys

lib/tallty_import_export/export.rb

@@ -106,7 +106,7 @@ module TalltyImportExport
       title1 = workbook.styles.add_style(alignment: alignment, border: border, sz: 12, b: true)
       title2 = workbook.styles.add_style(alignment: alignment, border: border, bg_color: '2a5caa', sz: 12, fg_color: 'fffffb')
       title3 = workbook.styles.add_style(alignment: alignment.merge(wrap_text: true), border: border, sz: 10)
-      headers = export_headers || export_headers_result(**options)
+      headers = (export_headers || export_headers_result(**options)).deep_symbolize_keys
 
       _sheet_name = respond_to?(:sheet_name) ? sheet_name : nil
       _sheet_name ||= sheet_name