http: close connection on 204 and chunked encoding

See [this comment](https://github.com/joyent/node/pull/4647#issuecomment-12581057).

This is similar to commit 2cbf458 but this time for 204 No Content
instead of 304 Not Modified responses.

When the user sends a 204 response with a Transfer-Encoding: chunked
header, suppress sending the zero chunk and force the connection to
close.

Suggested reviewers: @isaacs @pquerna