facebook/react — pull request #1198 by chenglou (2014-02-28 20:52:09+00:00)
Title: Allow className to be an array of strings

FIxes #1179.
`className={['a', null, 'b']}` becomes 'a b'.
This allows you to do `className={['a'].concat(this.props.className)}`.
Note that nested arrays are currently forbidden.