title: Failed prop type: Invalid prop openIcon supplied to CardHeader, expected a ReactNode
body: <p>I am trying to use Material-UI and create a CardHeader with custom open and close icons (openIcon, closeIcon).  I have tried sever different approaches to create the custom icons.  However I keep getting the error: <code>Failed prop type: Invalid prop openIcon supplied to CardHeader, expected a ReactNode</code>.  I have been searching for documentation identifying how to define openIcon and closeIcon however i have not been able to find anything.  I have tried SvgIcon, FontIcon and both produce the same error.  I have have seen the information on the Card <a href="http://www.material-ui.com/#/components/card" rel="nofollow noreferrer">here</a>. however it only specifies "node" with no examples or further definitions.  Could someone point me in the right direction as to where this is documented.</p>