Bug: A missing key and a key with a value of undefined are not considered the same

Example:
```typescript
interface Foo {
  foo?: string;
}

const foo1: Foo = {
  foo: undefined,
}

const foo2: Foo = {}

console.log(is<Foo>(foo1)); // false
console.log(is<Foo>(foo2)); // true
```

Should they not both be true?