[FlatButton] Fix Icon color prop issue

Closes #4144

Author: tintin1343

src/FlatButton/FlatButton.js

@@ -221,7 +221,7 @@ class FlatButton extends Component {
 
     if (icon) {
       iconCloned = React.cloneElement(icon, {
-        color: mergedRootStyles.color,
+        color: icon.props.color,
         style: {
           verticalAlign: 'middle',
           marginLeft: label && labelPosition !== 'before' ? 12 : 0,