querystring: parse numbers correctly

Fixes a number parsing regression introduced in 85a92a37

Fixes: https://github.com/iojs/io.js/issues/1208
PR-URL: https://github.com/iojs/io.js/pull/1213


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Brian White <mscdex@mscdex.net>