Deserializing a flattened struct containing a `ByteBuf` fails to deserialize with invalid UTF-8.

As per the comment for deserialize_bytes, it is expected that you can deserialize a non-UTF-8 string into a ByteBuf without failure. However, with flattened structures, deserialize_map is called in place of deserialize_struct (see https://github.com/serde-rs/serde/issues/1529), meaning members are deserialized with deserialize_any bypassing deserialize_bytes. deserialize_any assumes values surrounded by quotation marks are valid UTF-8 strings and returns an error otherwise.

An example follows

use serde::{Deserialize, Serialize};

fn main() {
    let a_success: A = serde_json::from_slice(b"{\"b\": {\"buf\": \"\xe5\x00\xe5\"}}").unwrap();
    println!("A Success: {:?}", a_success);

    let a_flat_success: AFlat = serde_json::from_slice(b"{\"buf\": \"abc\"}").unwrap();
    println!("A Flat Success: {:?}", a_flat_success);
    
    let a_flat_failure: AFlat = serde_json::from_slice(b"{\"buf\": \"\xe5\x00\xe5\"}").unwrap();
}

#[derive(Debug, Deserialize, Serialize)]
struct A {
    b: B,
}

#[derive(Debug, Deserialize, Serialize)]
struct AFlat {
    #[serde(flatten)]
    b: B,
}

#[derive(Debug, Deserialize, Serialize)]
struct B {
    buf: serde_bytes::ByteBuf,
}

Reproduce the example with A, AFlat, and serde_bytes::ByteBuf, then trace how flattened fields move through deserialize_map and deserialize_any instead of deserialize_struct and deserialize_bytes. Compare the behavior with the existing deserialize_bytes expectation; done means the flattened non-UTF-8 ByteBuf example deserializes successfully without regressing the non-flattened case.