js_stream: fix buffer index in DoWrite

The index of buffer to write in JSStream was always 0 by mistake. This
fix was to use increment index of buffer arrays.
The test was originally made by Brian White in #1594.

Fix: https://github.com/iojs/io.js/issues/1595
Fix: https://github.com/iojs/io.js/pull/1594
PR-URL: https://github.com/iojs/io.js/pull/1635


Reviewed-By: Fedor Indutny <fedor@indutny.com>