passing explicit undefined as className throws error

[amiralies]

I think this should be cheked in this line

[robmadole]

Just checked and the classnames NPM package handles this just fine. So I think there is some precedence for allowing undefined. Good catch. We'll get this on the list to get fixed. If anyone would like to see it faster than we can get to it PRs are welcome!

[charles4221]

This is fixed as of #581 (3.0.0)