deal with an empty response body from hackney

I tried to use httposion for cached get requests using  the "If-Modified-Since" header. Problem is that I get MatchError from within httpoison as soon as I expect a 304 response.

I looked though the source of httpoison and discovered that it can't deal when hackney returns an error code for an empty response body on a get request.

I didn't want to just open an issue although I'm not sure this is the best way to fix this as I'm still trying to get a feel for elixir. Maybe this should even be handled differently in hackney seeing that it does the correct thing if I do a :head request with the same header.

I also could not figure out how the tests are setup and how I could produce such a response in the tests.