5.0.2 strict breaks stringifySrcset with valid width

Hi!! I've found the version 5.0.2 in strict mode breaks a valid stringifySrcset:

Simple example:

```js
stringifySrcset([{ url: "banner-phone.jpeg", width: 100 }], { strict: true });
```

Gives an error in 5.0.2 but not in 5.0.1.

Thank you!