`undefined` becomes `null` after deserializing

mischnic

const msgpack = require('@msgpack/msgpack');

let original = {x: undefined};
let serialized = msgpack.encode(original, {extensionCodec});
let deserialized = msgpack.decode(serialized, {extensionCodec});
console.log(original, deserialized, original.x === deserialized.x);

prints "{ x: undefined } { x: null } false". This is a problem because now code checking for strict equality fails.

I was also not able to handle this in an extension codec because that is only called for non-primitive values.

ignoreUndefined: true doesn't work in all cases:

let original = undefined;
let serialized = msgpack.encode(original, {ignoreUndefined: true});
let deserialized = msgpack.decode(serialized, {ignoreUndefined: true});
console.log(original, deserialized, original === deserialized); // undefined null false

This is stated in the readme, but is there no way to retain this with an extension codec?