Merge pull request mui#2021 from callemall/oliviertassinari-patch-1

[Overlay] Fix server side rendering

Author: oliviertassinari

src/overlay.jsx

@@ -54,7 +54,7 @@ const Overlay = React.createClass({
     };
   },
 
-  componentWillMount() {
+  componentDidMount() {
     this._originalBodyOverflow = document.getElementsByTagName('body')[0].style.overflow;
   },