title: React Native: VirtualizedList only rendering 10 items
body: <p>Why does this VirtualizedList only render the first 10 items when it should be rendering 365? If I pass a <code>data</code> variable with more than 10 items it works fine, but fails when I use the <code>getItem</code> method. </p>
  
  <pre><code>export default class ListTest extends Component {
    render() {
      return (
        &lt;View&gt;
          &lt;VirtualizedList
            initialNumberToRender={20}
            windowSize={21}
            getItemCount={(data) =&gt; 365}
            getItem={(data, index) =&gt; {
              return { key: index };
            }}
            keyExtractor={(item, index) =&gt; {
              return item.key;
            }}
            renderItem={({ item, index }) =&gt; {
              return (
                &lt;View style={{height:50}}&gt;
                  &lt;Text&gt;{item.key}&lt;/Text&gt;
                &lt;/View&gt;
              );
            }}
          /&gt;
        &lt;/View&gt;
      );
    }
  }
  </code></pre>