Bugfix/through respect sti name

When deriving the value to look for in a polymorphic through relationship, this will only by default be the name of the base ActiveRecord class. This is handled by and may be overridden with the `sti_name` class method of the model.

https://github.com/rails/rails/commit/03118bc5afe99aaaf463a834d8a1ae16c918f6e8#commitcomment-7478184