\ characters should be escaped in paths of special URLs

[sfackler]

\ characters are not escaped when appended to paths currently:

let mut url = "http://foobar.com".parse::<Url>().unwrap();
url.path_segments_mut().unwrap().push("foo\\bar");
assert_eq!(url.as_str(), "http://foobar.com/foo\\bar");

However, the path-state section of the URL standard treats \ as a path separator and a validation error in special URLs. I believe push should be percent-encoding the \ to %5C.

[sfackler]

This causes some extra fun, since the parser does handle \ properly, so a URL constructed via push has a different number of path components than the same URL reparsed: https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=288a9f492fdd1bcb9c2afaefa7ebca5b