furrr_options() drops attributes on 'chunk_size' if it's non-integer

Attribute `ordering` is dropped when `chunk_size` is non-integer:

```r
> options <- furrr:::furrr_options(chunk_size = structure(4, ordering = "random"))
> options$chunk_size
[1] 4
```

Works as expected when the value is an integer:

```r
> options <- furrr:::furrr_options(chunk_size = structure(4L, ordering = "random"))
s> options$chunk_size
[1] 4
attr(,"ordering")
[1] "random"
```

