Support the Rack hijack API

[tarcieri]

Rack has an API that allows the socket to be hijacked from the web server and managed directly by the Rack application itself:

rack/rack#481

This allows certain problematic restrictions of Rack, such as mandatory rewindable input, to be bypassed. This makes it useful for things like WebSockets, server sent events, and streaming HTTP requests/responses.

Reel::Rack could certainly benefit from this API! In fact, Reel already has support for hijacking the connection. All that needs to be done is wiring up one with the other:

https://github.com/celluloid/reel/blob/master/lib/reel/connection.rb#L149