Restore :class_name in fixture

This one is fine, thanks to @skipkayhil for pointing this one out.

Author: fxn

activerecord/test/models/cpk/comment.rb

@@ -4,6 +4,6 @@ module Cpk
   class Comment < ActiveRecord::Base
     self.table_name = :cpk_comments
     belongs_to :commentable, foreign_key: %i[commentable_title commentable_author], polymorphic: true
-    belongs_to :post, foreign_key: %i[commentable_title commentable_author]
+    belongs_to :post, class_name: "Cpk::Post", foreign_key: %i[commentable_title commentable_author]
   end
 end