Rust bug: number keys are not properly handled

[ahdinosaur]

failing test:

macro_rules! treemap {
    () => {
        BTreeMap::new()
    };
    ($($k:expr => $v:expr),+) => {
        {
            let mut m = BTreeMap::new();
            $(
                m.insert($k, $v);
            )+
            m
        }
    };
}

#[test]
fn test_object_with_wacky_keys() {
    #[derive(PartialEq, Eq, PartialOrd, Ord, serde_derive::Serialize)]
    #[serde(untagged)]
    enum Key<'a> {
        Str(&'a str),
        Num(u32),
    }
    let expected = r#"{"\n":"Newline","1":"One","2":"Two","3":"Three","4":"Four"}"#;
    let input = treemap![
        Key::Num(2) => "Two",
        Key::Str("4") => "Four",
        Key::Str("1") => "One",
        Key::Num(3) => "Three",
        Key::Str("\u{000a}") => "Newline"
    ];
    let actual = to_string(&input).unwrap();
    assert_eq!(actual, expected);
}

failures:

---- test_object_with_wacky_keys stdout ----
thread 'test_object_with_wacky_keys' panicked at 'assertion failed: `(left == right)`
  left: `"{\"2\":\"Two\",\"3\":\"Three\",\"\\n\":\"Newline\",\"1\":\"One\",\"4\":\"Four\"}"`,
 right: `"{\"\\n\":\"Newline\",\"1\":\"One\",\"2\":\"Two\",\"3\":\"Three\",\"4\":\"Four\"}"`', 

i'll admit i didn't realize numbers could be keys, but that's because i keep thinking that the Rust is deserialized to serde_json::Value (where keys can only be strings) before being serialized, which is just not the case.