fix: common model_headers use klass

lib/tallty_import_export/common.rb

@@ -6,14 +6,14 @@ module TalltyImportExport
     end
 
     module ClassMethods
-      def model_headers **args
-        columns.map do |column|
+      def model_headers(**args)
+        klass.columns.map do |column|
           {
             key: column.name,
             name: column.comment || column.name,
             attr_type: column.type,
             format: column.type == :string ? :string : nil,
-            primary_key: column.name == 'id' ? true : false
+            primary_key: column.name == 'id',
           }
         end
       end