Fix werkzeug.urls

Fix #582

- [ ] How should ``file://home/untitaker/`` be converted? My intuition would be
  ``file:///home/untitaker/``. Firefox is no help here, its urlparsing
  apparently first recognizes "home" as a host, then sees that it's a
  file-URL... and discards the host, redirecting to ``file:///untitaker/``.
  Chromium shows an error for ``file://`` and only accepts the version with
  three slashes. (tested on Linux)