Created by: thiagobustamante
This PR fix "Can't set headers after they are sent" errors.
That are a lot of situations where this error can occurs. In my case, it is happening because I have others middlewares (in an expressjs application that tries to proxy requests). Some of those middlewares (like passportjs, or cors) can run res.end() and when the proxy receive a response, it is already finished.
So, it is necessary to test if we can write on the user response when the proxy response is ready.
I think it could also fix #930 (closed), #1168, #908 and #867