Attribute list parsing with serde does not treat `\t` as separator

[reknih]

Deserialization of a list in an attribute value does not treat the tab character as a separator, despite the documentation stating that it should.

use quick_xml::de::Deserializer;
use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct Any {
    #[serde(rename = "@list")]
    list: Vec<Item>,
}

#[derive(Debug, PartialEq, Deserialize)]
#[serde(rename_all = "lowercase")]
enum Item {
    Foo,
    Bar,
}

#[test]
fn main() {
    // The separator between the two characters is a tab.
    let str = r#"<any list="foo	bar" />"#;
    let mut deser = Deserializer::from_str(str);
    let any = Any::deserialize(&mut deser).unwrap();
    assert_eq!(any.list, vec![Item::Foo, Item::Bar]);
}

Expected behavior: The assertion passes.
Actual behavior: thread 'main' panicked at 'called Result::unwrap() on an Err value: Custom("unknown variant foo\tbar, expected foo or bar")'

[Mingun]

This is because we do not normalize attribute values according to the specification yet, where all whitespace characters would be turned into space, but deserializer already written with that in mind.