Options for Httpoison and Hackney to not append library headers?

Hi @hazardfn

I got a PR going to add an option to disable library headers, it’s the result of fiddling with code in /deps. I have very little experience with erlang, and can’t seem to figure out how to get the test suite to run. If it is not too much trouble, can you take a look at the PR, and point me in some general direction on how to properly test it?

Thank you!