Add a resp_body accessor.

i'm writing my first cowboy app. i need to modify resp_body to protect from CSRF.
In my way, modifying resp_body needs to get http_req.resp_body after setting it.

First, i wrote it by using cowboy_req:get/2, but it is private. Then i think the cowboy_req needs an accessor for the public.

Or, does there exist some more smart ways?

i have an example in a branch. That is;
https://github.com/kuniyoshi/cowboy/tree/acsrf_example

thanks.