Allow changing transitionLeave from false to true

Fixes #724.

src/addons/transitions/ReactTransitionGroup.js

@@ -79,6 +79,15 @@ var ReactTransitionGroup = React.createClass({
           enter: this.props.transitionEnter,
           onDoneLeaving: this._handleDoneLeaving.bind(this, key)
         }, childMapping[key]);
+      } else {
+        // If there's no leave transition and the child has been removed from
+        // the source children list, we want to remove it immediately from the
+        // _transitionGroupCurrentKeys cache because _handleDoneLeaving won't
+        // be called. In normal cases, this prevents a small memory leak; in
+        // the case of switching transitionLeave from false to true, it
+        // prevents a confusing bug where ReactTransitionableChild.render()
+        // returns nothing, throwing an error.
+        delete currentKeys[key];
       }
     }
 

src/addons/transitions/__tests__/ReactTransitionGroup-test.js

@@ -83,6 +83,41 @@ describe('ReactTransitionGroup', function() {
     expect(a.getDOMNode().childNodes[1].id).toBe('one');
   });
 
+  it('should switch transitionLeave from false to true', function() {
+    var a = React.renderComponent(
+      <ReactTransitionGroup
+          transitionName="yolo"
+          transitionEnter={false}
+          transitionLeave={false}>
+        <span key="one" id="one" />
+      </ReactTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(1);
+    React.renderComponent(
+      <ReactTransitionGroup
+          transitionName="yolo"
+          transitionEnter={false}
+          transitionLeave={false}>
+        <span key="two" id="two" />
+      </ReactTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(1);
+    React.renderComponent(
+      <ReactTransitionGroup
+          transitionName="yolo"
+          transitionEnter={false}
+          transitionLeave={true}>
+        <span key="three" id="three" />
+      </ReactTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(2);
+    expect(a.getDOMNode().childNodes[0].id).toBe('three');
+    expect(a.getDOMNode().childNodes[1].id).toBe('two');
+  });
+
   describe('with an undefined child', function () {
     it('should fail silently', function () {
       React.renderComponent(