Update the headers from prerender's response before returning to the client

It seems as though Prerender started gzipping their responses after this project was created. The `text` field of Parse's `HTTPResponse` object provides an unzipped version of the content but it doesn't update the `Content-Encoding` header, causing the client's browser to complain that the response isn't gzipped and thus can't read it. I looked at the most recent version of Prerender's official Node middleware and it has a function that simply unzips the response and deletes the `Content-Encoding` and `Content-Length` headers, so I made the same change to this project and everything seems to be working again.