Socket closed unexpectedly for chunked response with "Connection: close" request

``Reel::Connection#respond`` has not handled the response state correctly so that the socket was closed right after sending response headers when the request has ``Connection: close`` header.  It works perfectly for keep-alive request though.

```
request = Reel::Connection.new(socket).request 
request.respond :ok, :transfer_encoding => :chunked

 #  "Reel::SocketError: closed stream" raised here.
 request << "Hello" 
 request << "World"
 request.finish_response
```