Array::sort_by_key doesn't work with Value::as_* methods

[calavera]

The new Array::sort_by_key doesn't seem to work when combined with the Value::as_* methods that the crate provides, like Value::as_str. Since I'm guessing that's the most common way to use that sorting function, I'd expect this test below to work, but it doesn't compile:

    #[test]
    fn sorting_with_references() {
        let values = vec!["foo", "qux", "bar"];
        let mut array = Array::from_iter(values);
        array.sort_by_key(Value::as_str);
    }

Compilation error:

error[E0308]: mismatched types
   --> crates/toml_edit/src/array.rs:433:9
    |
433 |         array.sort_by_key(Value::as_str);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
    = note: expected enum `Option<&str>`
               found enum `Option<&str>`
note: the lifetime requirement is introduced here
   --> crates/toml_edit/src/array.rs:327:29
    |
327 |         F: FnMut(&Value) -> K,
    |                             ^

For more information about this error, try `rustc --explain E0308`.