integration_test#post returns 422 response

Reproduction instructions:

ruby script/generate scaffold cart
rake db:migrate
rake db:test:prepare

ruby script/generate integration_test cart_stories
sed -i -e 's/assert true/post "carts"; assert_response :found/' \
  test/integration/cart_stories_test.rb
ruby -I test test/integration/cart_stories_test.rb

Test passes in versions of Rails prior to http://github.com/rails/rails/commit/38aeb1528c376f7a058beea6db0a32...

Current results:

Loaded suite test/integration/cart_stories_test
Started
F

  1) Failure:
test_the_truth(CartStoriesTest) [test/integration/cart_stories_test.rb:8]:
Expected response to be a <:found>, but was <422>

Finished in 0.402899354 seconds.

1 tests, 1 assertions, 1 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
0% passed