The incorrect implementation of `set_scheme` for `file://`

[baivoom]

• [ x] Note that this crate implements the URL Standard not RFC 1738 or RFC 3986

Describe the bug

The scheme file will always have authority

but to set_scheme to file does not allow the file url to have the authority

Therefore the following test will fail in this case

#[test]
    fn test_url() {
        
        {
            let mut url = Url::parse("file:///path/to/file.js").unwrap();
            url.set_scheme("file").unwrap();
        }
    }

Not sure if it is a bug or a intentional design

[lwz23]

It seems like a Rust Bug problem, I am interested in it and want try to solve this problem.

[valenting]

@lwz23 feel free to submit a PR