Some style problems, IMHO:
* may you move this tests near https://github.com/iantropov/rails/blob/1ef6f4b0b1bfeb941e2115bb891465d7e564ed84/activerecord/test/cases/associations/has_many_through_associations_test.rb#L48
* use `def test_` synta
* and also describe in test name that you test `create` action with STI association

I'll review your patch tomorrow