Request rendered pages from prerender with gzip encoding

This is meant to be used with the gzip feature for which I made a pull request in the prerender project.

With the entire chain being in gzip, I see a response time of about 150 ms with gzip content in a nodejs database. This is compared to about 800 ms that I saw without any gzip. So big gains here.

Both the requesting server using prerender-node and the prerender server are running in different heroku apps using 1 dyno.