url-constructor: '..' against a non-special base must pop the last path segment

[nikneym]

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

rust-url tries to remove the trailing slash, but last_slash_can_be_removed("/y/z/C:/") sees the last dir is a drive letter (/C:/), as a consequence returns false, the trailing slash stays. You can reproduce with the following snippet.

use url::Url;

fn join(base: &str, rel: &str) -> String {
    Url::parse(base).unwrap().join(rel).unwrap().into()
}

fn main() {
    // A path segment "C:" is only special for file: URLs (WHATWG URL Standard).
    // For any other scheme it is an ordinary segment, so ".." must pop it.

    // Non-special scheme:
    assert_eq!(join("abc://x/y/z/C:/", ".."), "abc://x/y/z/");
    // Special, non-file scheme — same bug:
    assert_eq!(join("http://x/y/z/C:/", ".."), "http://x/y/z/");
    // The "C|" (pipe) form is affected too:
    assert_eq!(join("abc://x/y/z/C|/", ".."), "abc://x/y/z/");

    // Controls that already behave correctly (ordinary segments pop):
    assert_eq!(join("abc://x/y/z/w/",  ".."), "abc://x/y/z/");
    assert_eq!(join("abc://x/y/z/Ca/", ".."), "abc://x/y/z/");
    // And a drive-letter segment WITHOUT a trailing slash pops fine:
    assert_eq!(join("abc://x/y/z/C:",  ".."), "abc://x/y/");

    println!("ok");
}