Attempt at handling ETag / If-None-Match on proxied content. I plan on using this to front a Varnish server using a lot of ESI. It does kind of work, but... be aware, this is my first attempt at developping a nginx plugin, and dealing with headers after having read the body was not exactly in the how-to. Any comment and/or improvement and/or fork is welcome. Thanks to http://github.com/kkung/nginx-static-etags/ for... inspiration.