Fix for the latest http gem.

tarcieri/http#80 introduced new API and it interferes Reel to stop with syntax error.

```
/home/travis/.rvm/rubies/ruby-2.0.0-p353/bin/ruby -S rspec ./spec/reel/connection_spec.rb ./spec/reel/http_server_spec.rb ./spec/reel/https_server_spec.rb ./spec/reel/response/writer_spec.rb ./spec/reel/response_spec.rb ./spec/reel/websocket_spec.rb
[Coveralls] Set up the SimpleCov formatter.
[Coveralls] Using SimpleCov's default settings.
/home/travis/build/unakatsuo/reel/lib/reel/response.rb:5:in `<class:Response>': uninitialized constant HTTP::Header (NameError)
    from /home/travis/build/unakatsuo/reel/lib/reel/response.rb:4:in `<module:Reel>'
    from /home/travis/build/unakatsuo/reel/lib/reel/response.rb:3:in `<top (required)>'
```

 This PR fixes the API compatibility issue in Reel::Response.