Don't replace spaces for %20 in URLs

The markdown parser from John Gruber doesn't do this, and there is no mention of this in the specification.

This is a broken implementation because it doesn't encode everything. Ie, there may already be a `%20` in the URL and a space. The original `%20` should become `%2520` and the space would already be turned into `%20`. If this should be percent encoding the URL's then it should use `urllib.quote` so it handles these cases and also encodes every other character.