Joining relative path onto a file:// url with a hostname is non spec compliant

use url::Url;
fn main() {
    let u = Url::parse("file://server/some/dir/file")
        .unwrap()
        .join("/another/path/at/the/same/server")
        .unwrap();
    println!("{}", u);
}

Expected output: file://server/another/path/at/the/same/server
Actual output file:///another/path/at/the/same/server

Compare also the output from Firefox developer tools:

> new URL("/another/path/at/the/same/server", "file://server/some/dir/file")
URL {href: "file://server/another/path/at/the/same/server", origin: "file://", protocol: "file:", username: "", password: "", …}

Start at the Url::join entry point shown in the reproduction and trace how file:// URLs with a hostname are handled. Compare the result with the expected URL and the Firefox behavior, then add a regression test covering the reported join and verify the test suite passes.