fix: make_relative missing filename when filenames equal in both urls

[dsherret]

let from_path = url::Url:::parse("http://127.0.0.1:8080/file.txt").unwrap();
let to_path = url::Url:::parse("http://127.0.0.1:8080/test/file.txt").unwrap();
println!("{}", from_path.make_relative(&to_path).unwrap());

Expected: "test/file.txt"
Actual: "test"

Not sure if this expected is correct or if the solution is right, but it passes the tests.

[jgraettinger]

We hit this bug too. The current behavior is incorrect, and this PR LGTM for fixing it. Can it be merged?

[jgraettinger]

/cc @valenting this is a verified fix for a still-present bug in Url::make_relative. Can it be merged? Thanks.

[valenting]

Thanks for the ping. This looks good to me.