Add cowboy_req:get_resp_body/1.

There is no http_req record exported, but i want to get
resp_body to modify it.

In my case, that will need when a app needs Anti CSRF.
My Anti CSRF adds `<input type="hidden" name="acsrf" value="xxx">`
to the end of each form.

i do that by using cowboy_middleware, and need to get resp_body to
modify it.  My middleware gets resp_body, adds `input` tag, and,
reply it.
Merge branch 'add_getter_to_cowboy_req' of ssh+git://github.com/kuniyoshi/cowboy into add_getter_to_cowboy_req