title: React-navigation: header does not show up when using bottom tab navigator
body: <p>I am using <code>react-navigation</code> with my react native app. I have created a bottom tab navigator, and want to use the built in header on my screen. But the header is not showing up. There are no errors or warnings. </p>
  
  <p>app.js:</p>
  
  <pre><code>const TabStack = createBottomTabNavigator({
    Upload: { 
      screen: upload,
      navigationOption: {
        headerTitle: "Upload"
      }
    },
    Profile: { 
      screen: profile,
      navigationOption: {
        headerTitle: "Profile"
      }
    }
  });
  
  const MainStack = createSwitchNavigator(
    {
      Home: { screen: TabStack }
    },
    {
      initialRouteName: 'Home'
    }
  );
  </code></pre>
  
  <p>upload.js</p>
  
  <pre><code>class upload extends React.Component {
      static navigationOptions = {
          title: 'Upload'
      };
  
      constructor(props) {
          super(props);
  
      ...
  </code></pre>
  
  <p>I know declaring <code>navigationOptions</code> in the components is probably not needed since it is already declared in app.js but this is just to show that neither approach works. </p>
  
  <p>How do I fix this?</p>