http: send Content-Length when possible

This changes the behavior for http to send send a Content-Length header
instead of using chunked encoding when we know the size of the body when
sending the headers.

Fixes: https://github.com/iojs/io.js/issues/1044
PR-URL: https://github.com/iojs/io.js/pull/1062


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Brian White <mscdex@mscdex.net>