Resolve default prop values in cloneElement

In cloneElement, when key in input config is set to undefined, the associated
prop value should be resolved to default prop values

src/isomorphic/classic/element/ReactElement.js

@@ -243,10 +243,19 @@ ReactElement.cloneElement = function(element, config, children) {
       key = '' + config.key;
     }
     // Remaining properties override existing props
+    var defaultProps;
+    if (element.type && element.type.defaultProps) {
+      defaultProps = element.type.defaultProps;
+    }
     for (propName in config) {
       if (config.hasOwnProperty(propName) &&
           !RESERVED_PROPS.hasOwnProperty(propName)) {
-        props[propName] = config[propName];
+        if (config[propName] === undefined && defaultProps !== undefined) {
+          // Resolve default props
+          props[propName] = defaultProps[propName];
+        } else {
+          props[propName] = config[propName];
+        }
       }
     }
   }

src/isomorphic/classic/element/__tests__/ReactElement-test.js

@@ -273,6 +273,29 @@ describe('ReactElement', function() {
     expect(inst2.props.prop).toBe(null);
   });
 
+  it('should normalize props with default values in cloning', function() {
+    var Component = React.createClass({
+      getDefaultProps: function() {
+        return {prop: 'testKey'};
+      },
+      render: function() {
+        return <span />;
+      },
+    });
+
+    var instance = React.createElement(Component);
+    var clonedInstance = React.cloneElement(instance, {prop: undefined});
+    expect(clonedInstance.props.prop).toBe('testKey');
+    var clonedInstance2 = React.cloneElement(instance, {prop: null});
+    expect(clonedInstance2.props.prop).toBe(null);
+
+    var instance2 = React.createElement(Component, {prop: 'newTestKey'});
+    var cloneInstance3 = React.cloneElement(instance2, {prop: undefined});
+    expect(cloneInstance3.props.prop).toBe('testKey');
+    var cloneInstance4 = React.cloneElement(instance2, {});
+    expect(cloneInstance4.props.prop).toBe('newTestKey');
+  });
+
   it('throws when changing a prop (in dev) after element creation', function() {
     var Outer = React.createClass({
       render: function() {