Make ReasonPhrases compatible with codes as keys.

Currently, we can use `StatusCodes` similar to a typescript enum like so:
```js
console.log(StatusCodes.GONE); //411
console.log(StatusCodes[411]); //"Gone"
```

but this does not work with `ReasonPhrases`:
```js
console.log(ReasonPhrases.GONE); //"Gone"
console.log(ReasonPhrases[411]); //undefined
```

We should be able to use `ReasonPhrases[code]`