fix column merge use col_index

lib/tallty_import_export/export.rb

@@ -90,7 +90,7 @@ module TalltyImportExport
           row = []
           headers.each_with_index do |header, col_index|
             _data = handle_data(record, header, index)
-            if header[:merge].present? && last_row.present? && _data == last_row[index]
+            if header[:merge].present? && last_row.present? && _data == last_row[col_index]
               merge_index = merge_column_hash[col_index] || []
               merge_index << index + first_content_row_index
               merge_index << index + first_content_row_index + 1