querystring: fix broken stringifyPrimitive

stringifyPrimitive has always failed to stringify numbers since its
introduction in 422d3c93. This went uncaught due to encodeURIComponent's
string coercion.

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>