Pin: Really pass special headers

My previous commit around this functionality was actually not passing
the special headers like it was supposed to.
(5dc3bd5c78470068eda1af098e14d6d255f6fa82)

This commit fixes the previous careless attempt and adjusts things such
that we're now actually testing that the right headers get passed rather
than just testing that the right headers are created.

Closes #654.