Leading slashes are dropped while setting pathname for file urls

[kt3k]

use url::Url;

fn main() {
    let mut u = Url::parse("file:///").unwrap();
    u.set_path("//foo");
    println!("{}", u);
}

This should prints file:////foo instead of file:///foo.

Note: The below prints file:////foo in all major browsers

const a = new URL("file:///");
a.pathname = "//foo";
console.log(a.href);

This behavior affects the WPT cases below:

<file://monkey/> set pathname to <\\\\>
<file:///unicorn> set pathname to <//\\/>
<file:///unicorn> set pathname to <//monkey/..//>