Add path_mut

[reem]

Add a path_mut method, which provides a mutable reference to the requested path. This is important for things like a Mounting middleware, which need to be able to modify the url.

[Valve]

+1, it's unclear how to update path after URL parsing. In my app I'm storing the base URL and updating the path depending on what I need to do.

This is what I'm currently doing to update the path:

fn update_url(baseUrl: &Url) -> Url{
  let mut url_parser = UrlParser::new();
  url_parser.base_url(baseUrl).parse("./update").unwrap()
}

[SimonSapin]

I find it very unfortunate that we have to duplicate accessors rather make them generic over mutability, but I guess that’s what we’re stuck with for now.

[Valve]

Thank you