streams: use strict on _stream_wrap

A Mostly Harmless™ change to enable 'use strict' mode in _stream_wrap, bringing it in line with /all/ the other modules.

PR-URL: https://github.com/iojs/io.js/pull/1279


Reviewed-By: Brian White (@mscdex) <mscdex@mscdex.net>
Reviewed-By: Roman Reiss (@silverwind) <me@silverwind.io>
Reviewed-By: Yosuke Furukawa <(@yosuke-furukawa)>
<yosuke.furukawa@gmail.com>