Stringify value in ReactDOMInput / ChangeEventPlugin

This fixes two bugs related to string-casting in React:

 # Setting `<input value={0} />` would use an empty `value` because `0` is falsey.
 # Using `onChange` and `setState` with non-strings could lead to an infinite loop.

The latter is possible with controlled inputs when:

 - User changes input value.
 - `onpropertychange` fires.
 - `ChangeEventPlugin` dispatches `onChange`.
 - A handler responds via `this.setState` with a non-string value (e.g. a number).
 - The input re-renders and re-sets `value`.
 - The new `value` is not a string, but the current `value` (read from the element) is cast to a string automatically by the browser.
 - This triggers another `onpropertychange`.
 - `ChangeEventPlugin` dispatches another `onChange`.
 - ...

src/dom/components/ReactDOMInput.js

@@ -62,7 +62,8 @@ var ReactDOMInput = ReactCompositeComponent.createClass({
   },
 
   getValue: function() {
-    return this.props.value != null ? this.props.value : this.state.value;
+    // Cast `this.props.value` to a string so equality checks pass.
+    return this.props.value != null ? '' + this.props.value : this.state.value;
   },
 
   render: function() {
@@ -85,10 +86,12 @@ var ReactDOMInput = ReactCompositeComponent.createClass({
       );
     }
     if (this.props.value != null) {
+      // Cast `this.props.value` to a string so falsey values that cast to
+      // truthy strings are not ignored.
       DOMPropertyOperations.setValueForProperty(
         rootNode,
         'value',
-        this.props.value || ''
+        '' + this.props.value || ''
       );
     }
   },

src/eventPlugins/ChangeEventPlugin.js

@@ -169,7 +169,8 @@ var newValueProp =  {
     return activeElementValueProp.get.call(this);
   },
   set: function(val) {
-    activeElementValue = val;
+    // Cast to a string so we can do equality checks.
+    activeElementValue = '' + val;
     activeElementValueProp.set.call(this, val);
   }
 };
@@ -231,7 +232,7 @@ function handlePropertyChange(nativeEvent) {
 /**
  * If a `change` event should be fired, returns the target's ID.
  */
-function  getTargetIDForInputEvent(
+function getTargetIDForInputEvent(
     topLevelType,
     topLevelTarget,
     topLevelTargetID) {