Humanize doesn't work with ISO 8601 strings

Great library, thanks for the work!

I'm having trouble understanding how I can get the same output from `.fromNow()` and `.humanize()`.

This works great:

```js
dayjs("2021-09-05T00:14:43.702Z").fromNow(); // outputs '2 months ago'
```

This doesn't:

```js
dayjs.duration("2021-09-05T00:14:43.702Z", "months").humanize(); // outputs 'NaN years'
```

The documentation isn't clear on what the first argument of `duration()` needs to be, but it looks like it doesn't like the date being in ISO 8601 format and perhaps wants a number instead?