Don't set DOM attributes to "undefined" on update

We already skip `null` and `undefined` when building up the stringified html on first render, but if you update a component to the *exact same* conditions, React will leave the DOM in a different state. We shouldn't do that.

src/core/ReactDOMIDOperations.js

@@ -71,7 +71,15 @@ var ReactDOMIDOperations = {
       'updatePropertyByID(...): %s',
       INVALID_PROPERTY_ERRORS[name]
     );
-    DOMPropertyOperations.setValueForProperty(node, name, value);
+
+    // If we're updating to null or undefined, we should remove the property
+    // from the DOM node instead of inadvertantly setting to a string. This
+    // brings us in line with the same behavior we have on initial render.
+    if (value != null) {
+      DOMPropertyOperations.setValueForProperty(node, name, value);
+    } else {
+      DOMPropertyOperations.deleteValueForProperty(node, name);
+    }
   },
 
   /**