ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

```
class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end
```

`author.posts.first.author` works correctly, but
`author.special_posts.first.author` does not work correctly.