allow iodata as the response body

Hi Jose

I asked few days ago on irc (nick hnc) for a way to pass iolist-formed bodies via Plug.Conn.send_resp/3 to Cowboy. Well I checked it an Cowboy allows passing iolists - please see cowboy_req:reply/4 at https://github.com/extend/cowboy/blob/master/src/cowboy_req.erl#L1009

So here's the Plug patch, I tested it with my webserver heavily using iolists, works fine.

Best,
Wojtek