stream: use nop as write() callback if omitted

This commit introduces a nop function that is used as the
Writable.prototype.write() callback when one is not provided.
This saves on function object creation.

PR-URL: https://github.com/iojs/io.js/pull/564


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>