Cannot set data-* attribute on VirtualDOM.

Hi,

I try to set data-\* attribute to element with no luck on [virtual-dom](https://github.com/Matt-Esch/virtual-dom)

For example:
This working:

``` js
hx`Next`
```

But this is not:

``` js
hx`Next`
```

I've googled and found this https://github.com/Matt-Esch/virtual-dom/blob/master/docs/vnode.md#custom-attributes-data-
which maybe related to my issue.