http_server: `prefinish` vs `finish`

Do not detach the socket from the response until all data is actually
sent to the other side.

See: https://github.com/iojs/io.js/pull/1373
PR-URL: https://github.com/nodejs/io.js/pull/1411


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>