Sanitize file URLs

[OzTK]

The current behaviour is creating a bad file url when the path is set to the root:

    server.datastore = new tus.FileStore({
      path: '/'
    })

results in a URL of the form http://localhost//12334456

This sanitizes the constructed url when posting the download so that it results in a valid URL for the created file. Trimming was added as well.