No longer percent encode spaces in urls.

The current implementation was wrong as it also percent encoded query strings
(which should be plus encoded) and calling urllib.quote on the path (and
urllib.quote_plus on the query string) assumes the url is not already encoded.
What if the document author pasted a url that was already encoded? She probably
did not intend for `%20` to become `%2520`. Or did she? It is now clear to me
why many implementation do nothing to urls. Just pass them though as-is. To bad
if they are not valid HTML. HTML authors have to encodee their own urls, so I
guess markdown authors have to as well.