Allow falsy values in statics

We found that the component would break if we set any statics with the
value 0. It turns out @chenglou already ran into this with
d71736b3ed but the statics code was copied
earlier, and still has this falsy check. Made the same change, updated
the unittest.

src/core/ReactCompositeComponent.js

@@ -503,7 +503,7 @@ function mixStaticSpecIntoComponent(ConvenienceConstructor, statics) {
   }
   for (var name in statics) {
     var property = statics[name];
-    if (!statics.hasOwnProperty(name) || !property) {
+    if (!statics.hasOwnProperty(name)) {
       return;
     }
 

src/core/__tests__/ReactCompositeComponent-test.js

@@ -1007,7 +1007,10 @@ describe('ReactCompositeComponent', function() {
   it('should support statics', function() {
     var Component = React.createClass({
       statics: {
-        abc: 'def'
+        abc: 'def',
+        def: 0,
+        ghi: null,
+        jkl: 'mno'
       },
 
       render: function() {
@@ -1018,6 +1021,12 @@ describe('ReactCompositeComponent', function() {
     ReactTestUtils.renderIntoDocument(instance);
     expect(instance.constructor.abc).toBe('def');
     expect(Component.abc).toBe('def');
+    expect(instance.constructor.def).toBe(0);
+    expect(Component.def).toBe(0);
+    expect(instance.constructor.ghi).toBe(null);
+    expect(Component.ghi).toBe(null);
+    expect(instance.constructor.jkl).toBe('mno');
+    expect(Component.jkl).toBe('mno');
   });
 
   it('should support statics in mixins', function() {