Checking options length

https://github.com/bitvavo/node-bitvavo-api/blame/70f300e74a69795b850c966136905db6844ce8ac/lib/utils.js#L10

As you said here:

```javascript
@param {{ [key: string]: string | number | boolean }} options
```

The type of `options` can be `string | number | boolean`, but is it possible to calculate the length of `number` or `boolean` ?
I think there is a possibility of error.