No classname will be generated from objects that have the "push" member set to true

In the following case all the classnames defined in the object are missing from the result:

```js
clsx('stack', { pop: true, push: true }); // => 'stack'
```

This is caused by the `if (!!mix.push) {` check for Array, this makes `clsx` so fast, but I think it should be replaced with `Array.isArray()` or at least add this case to readme to warn people that get into this issue.