Url::make_relative strips leading /

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

This is an arguably less-critical version of https://github.com/servo/rust-url/issues/766.

Leading slashes are also incorrectly stripped. For example, I would expect this to print "/bar":

use url::Url;

fn main() {
    let base = Url::parse("https://foo.com/").unwrap();
    let url = base.join("https://foo.com//bar").unwrap();
    println!("{}", base.make_relative(&url).unwrap());
    // actually prints "bar"
}

As it is, base.join("bar") obviously gives https://foo.com/bar which is a different url to https://foo.com//bar per the URL Standard.

Start at the Url::make_relative entry point and reproduce the example from the issue, comparing the result for https://foo.com//bar with the URL Standard behavior. Done means the relative result preserves the leading slash and a regression test covers this case.