[React::Children] Fix the spec

1. React::Children is considered to be a Enumberable, so only #count method should be defined.
2. React::Children#each will returns a Enumerator, with #size returned nil when cannot count lazily, and in our case, it could be counted lazily.

Author: zetachang

spec/react/component_spec.rb

@@ -710,15 +710,15 @@ def render
       children = Foo.the_children
 
       expect(children).to be_a(React::Children)
-      expect(children.size).to eq(2)
+      expect(children.count).to eq(2)
       expect(children.map(&:element_type)).to eq(['a', 'li'])
     end
 
     it 'returns an empty Enumerator if there are no children' do
       ele = React.create_element(Foo)
       renderElementToDocument(ele)
       nodes = Foo.the_children.each
-      expect(nodes.size).to eq(nil)
+      expect(nodes.size).to eq(0)
       expect(nodes.count).to eq(0)
     end
   end