Fix no method error with HTTP::Parser#request_path

http_parser removed `request_path` method. Now, kage is broken with latest version(0.6.0) of http_parser. 
So, it's better to parse `Http::Parser#request_url` with `URI.parse`.
Do you think about it?

[links]
- https://github.com/tmm1/http_parser.rb/pull/15
- https://github.com/igrigorik/em-websocket/blob/master/lib/em-websocket/handshake.rb#L88