Make has method a type guard

[yotamN]

While widening the has method helps, it still cause problems with cases where we want to access the data later:

if (map.has(key)) {
  map.get(key); // will fail since key was not narrowed
}

[mattpocock]

Please add tests, and once tests are added you'll notice the flaws in your implementation.

For instance, map.has(key) doesn't guarantee that key is type typeof key.