Sequences with 1 entry do not work if nested with #[serde(flatten)]

[liss-h]

Hi,
there seems to be an issue with the #[serde(flatten)] attribute, as it prevents sequences with 1 element from being properly parsed. (zero or more than one work fine)

Example:

use serde::Deserialize;

#[derive(Deserialize)]
struct Seq {
    #[serde(default)]
    seq: Vec<String>,
}

#[derive(Deserialize)]
struct WithFlatten {
    #[serde(flatten)]
    inner: Seq,
}

fn main() {
    { // this works as expected
        let just_seq: Seq = serde_html_form::from_str("seq=1").unwrap();
        assert_eq!(just_seq.seq, vec!["1".to_owned()]);
    }

    { // this fails on the unwrap()

         // called `Result::unwrap()` on an `Err` value: Error("invalid type: string \"1\", expected a sequence")
        let with_flatten: WithFlatten = serde_html_form::from_str("seq=1").unwrap(); 
        assert_eq!(with_flatten.inner.seq, vec!["1".to_owned()]);
    }
}

[jplatte]

This is an instance of serde-rs/serde#1183 and can't be fixed in thie library.

I can only strongly recommend to not use serde(flatten) when you can help it. Even when it works, it degrades performance quite a lot compared to the equivalent solution without flatten.