Use alists instead of dicts for StubTreq, so we don't need to worry about hashing

This allows us to test the value of kwargs (like log) if we want to.

Also get rid of the no longer necessary headers_to_hashable function.
Hahaha!  I was going to suggest checking only elements after the current element, but fair enough.  :)  the list is probably pretty small for the particular use case of StubTreq.  Maybe make this a private module function though, so it'd be likely to only be used by Stubtreq?