OptGroup className not working

[fgblomqvist]

According to the typescript type it supports classname (and it should, since it can be useful for styling). However, it doesn't seem to pass the classname through. Would submit a PR but I have no idea where the code is (src seems to just be types?) 🤔

[atom-sgt]

The documentation for OptGroup makes no mention of className as a valid prop. Worth noting that className does exist for Option. https://github.com/react-component/select#optgroup-props

I would like it to be added, but it does not seem to be there currently.