serde De/Serializing Option<u64> fails

[jonaspleyer]

Hi,

I have some code in which I need to (de)-serialize in the following way:

#[test]
fn issue785() {
    #[derive(Debug, Deserialize, Eq, PartialEq, Serialize)]
    struct Identifier {
        parent: Option<u64>,
    }
    let ident = Identifier {
        parent: None,
    };
    let mut save_string = String::new();
    let mut serializer = quick_xml::se::Serializer::new(&mut save_string);
    serializer.indent(' ', 4);
    ident.serialize(serializer).unwrap();
    let res: Identifier = quick_xml::de::from_str(&save_string).unwrap();
    assert_eq!(res, ident);
}

This test does however fail with the current implementation of quick-xml. I have narrowed it down to the deserialize_option function of the MapValueDeserializer struct in src/de/map.rs. Sadly, I do not understand enough about the internals of quick-xml to contribute anything meaningful. Please let know how I can further help if needed.

PS: I have created a PR #786 to work on this problem.

[Mingun]

Duplicate of #497