add export_headers method default

lib/tallty_import_export/export.rb

@@ -107,7 +107,11 @@ module TalltyImportExport
     end
 
     def export_headers_result
-      @headers || klass.try(:export_headers) || klass.try(:headers) || klass.try(:model_headers)
+      @headers || export_headers
+    end
+
+    def export_headers
+      @headers || klass.try(:headers) || klass.try(:model_headers)
     end
 
     # 处理一个记录的数据

lib/tallty_import_export/exportable.rb

@@ -16,10 +16,6 @@ module TalltyImportExport
       def export_xlsx records, **options
         export_instance.export_xlsx(records, **options)
       end
-
-      def export_headers
-        export_instance.export_headers
-      end
     end
   end
 end