Using integer as a map key causes incorrect rendering

[Kogia-sima]

Description

I'd like to serialize BTreeMap which has integer as a key. However, when I passed map into to_string or to_string_pretty, simd_json outpus invalid JSON data.

use std::collections::BTreeMap;

fn main() {
    let mut map = BTreeMap::new();
    map.insert(0, "foo");
    map.insert(1, "bar");
    map.insert(2, "baz");

    println!("{}", simd_json::to_string_pretty(&map).unwrap());
}

output:

{
  0: "foo",
  1: "bar",
  2: "baz"
}

Expected behavior

I want the JSON output like this:

{
  "0": "foo",
  "1": "bar",
  "2": "baz"
}

Possible solution

serde_json crate defines a new struct named MapKeySerializer to properly render the map keys.

Environment

Software	Version
OS	Lubuntu 20.04.2 LTS
rustc	rustc stable 1.48.0 (7eac88abb 2020-11-16)
simd-json	0.3.23