Callback support and filtered inappropriate headers from prerender service.

The latency added by sending an HTTP request to the prerender service for every single crawlable request is not scalable. I had a need to actual do the cache setting and getting from the middleware.

Previously I had monkey patched this behavior but I went ahead and added it to the prerender_rails gem.


I also fixed a bug in the latest version that would leave requests open and hanging due to Content-Length mis-matches. I went ahead and added a constant to filter out headers we don't actually want from the prerender service.

Let me know if this is something you'd like to merge and I can add documentation for the callbacks.