url: improve parsing speed

The url.parse() function now checks whether an escapable character is in
the URL before trying to escape it.

PR-URL: https://github.com/joyent/node/pull/8638
[trev.norris@gmail.com: Switch to use continue instead of if]
Signed-off-by: Trevor Norris <trev.norris@gmail.com>
querystring: improve parse() performance