title: React Native keyExtractor fails to function properly in SectionList
body: <p>Below is the keyExtractor method I'm using. This method worked fine when the component was a FlatList, but I keep getting the following message after I converted the FlatList to a SectionList.</p>
  
  <p>"Warning: VirtualizedSectionList: A <code>section</code> you supplied is missing the <code>key</code> property."</p>
  
  <pre><code>// defining the keyExtractor function
  
  _keyExtractor = (item, index) =&gt; {
      console.log('id in key extractor', item.id)
      return item.id;
    }
  
  // during render
  
  &lt;SectionList
     ...
     keyExtractor={this._keyExtractor}
     ...
  /&gt;
  </code></pre>
  
  <p>I'm logging the item.id and it's printing out distinct ids correctly. Any thoughts? Thanks in advance.</p>