[mml]

Cowboy is quite a well respected we server of the Erlang flavor. I'd guess heroku rejiggered something in their stack, perhaps adding cowboy as a reverse proxy or load balancer in front of their junk.

Cowboy apparently shot yor no-good dirty sidewinding web requests in the face.

[mqsiuser]

It is well known that you can't (should not) rely on bugs (or internal APIs)