firefox requires 'scroll' to be set ovtherwise setting scrollTop and scrollLeft does nothing (marcj#109)

Author: jsdw

src/ResizeSensor.js

@@ -118,7 +118,7 @@
 
             element.resizeSensor = document.createElement('div');
             element.resizeSensor.className = 'resize-sensor';
-            var style = 'position: absolute; left: 0; top: 0; right: 0; bottom: 0; overflow: hidden; z-index: -1; visibility: hidden;';
+            var style = 'position: absolute; left: 0; top: 0; right: 0; bottom: 0; overflow: scroll; z-index: -1; visibility: hidden;';
             var styleChild = 'position: absolute; left: 0; top: 0; transition: 0s;';
 
             element.resizeSensor.style.cssText = style;