Fix roundtrip of flatten externally tagged enum unit variant

[Mingun]

Unit variant of externally tagged enum can be deserialized when enum is flatten, but cannot be serialized -- ser test failed:

#![cfg(test)]

use serde::{Deserialize, Serialize}; // 1.0.204;
use serde_json::{from_str, to_string}; // 1.0.121

#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Flatten {
    #[serde(flatten)]
    data: Enum,
}

#[derive(Debug, PartialEq, Serialize, Deserialize)]
enum Enum {
    Unit,
}

const JSON: &str = r#"{"Unit":null}"#;

#[test]
fn de() {
    // ok
    assert_eq!(from_str::<Flatten>(JSON).unwrap(), Flatten { data: Enum::Unit });
}

#[test]
fn ser() {
    // called `Result::unwrap()` on an `Err` value: Error("can only flatten structs and maps (got an enum)", line: 0, column: 0)
    assert_eq!(to_string(&Flatten { data: Enum::Unit }).unwrap(), JSON);
}

This PR fixes this inconsistency

[Mingun]

This PR does not conflicts with #2608, because changed files not moved in #2608, it has pretty obvious test case and reproduction example, so is it seems it shouldn't take too much time for review.