Supporting string newtypes

[sabify]

I just found out that string newtypes are considered sequences with single element. Is it intentional?

serde_qs/src/de/mod.rs

Lines 629 to 633 in b7278b7

	Level::Flat(_) => {
	// For a newtype_struct, attempt to deserialize a flat value as a
	// single element sequence.
	visitor.visit_seq(LevelSeq(vec![self.0].into_iter()))
	}

Please consider the following case:

We define a newtype, in this case with nutype macro and we are going to use it as part of a struct:

use nutype::nutype;

#[nutype(
    sanitize(trim, lowercase),
    derive(Serialize, Deserialize)
)]
pub struct Email(String);

#[derive(Serialize, Deserialize)]
pub struct SignInReq {
    email: Email,
    // ...
}

Now when we are trying to deserialize a qs string, something like req[email][email protected], it returns error Err(Deserialization(Custom("invalid type: sequence, expected tuple struct Email"))).