Keep calculator solution up to date if unit test was changed.

[helloShen]

Keep calculator.js and calculator.spec.js up to date, if previous pull request #223 08_calculator add some unit tests was approved.

Need to add 1 line to factorial() and recursiveFactorial() solution, so they can pass all tests.

if (n < 0) return undefined;

All 28 tests passed.

---

In the second commit Add an alternative power() solution, I add 1 more solution for power().

The old power() solution simply just call the Math.pow().

const power = function(a, b) {
  return Math.pow(a, b);
};

I think the author is right. Considering that arguments can be any float number (ex: Math.pow(2, 2.5) = 5.65685424949238), it's not that easy to write a real world power() by ourself.

But why not give them an alternative naive example. If we assume the arguments are integers, it's quite easy.

I keep the old power() as a main solution. The alternative intPower() is listed behind as a showcase. All 28 unit tests passed.