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 The test was originally made by @mscdex in #1594.

CI results are https://jenkins-iojs.nodesource.com/job/iojs+any-pr+multi/646/ . Most of failures are caused by cpplint errors.

Fixes: https://github.com/iojs/io.js/issues/1595 Fixes: https://github.com/iojs/io.js/pull/1594

R= @indutny