querystring: don't stringify bad surrogate pair

Fixes: https://github.com/nodejs/node/issues/3702
PR-URL: https://github.com/nodejs/node/pull/5858


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>