Don't double-encode URLs before parsing them

<https://code.google.com/p/macvim/issues/detail?id=316> wasn't a bug.

When you're constructing a URL, you *should* end up escaping URL-unsafe
characters twice: once for the `file:` URL and once for the `mvim:` URL.
Escaping it inside MacVim breaks legit URLs with escapes at the `mvim:` level.