title: react-navigation dynamic header doesn't work?
body: <p>I followed exactly the tutorial <a href="https://reactnavigation.org/docs/intro/" rel="nofollow noreferrer">https://reactnavigation.org/docs/intro/</a>
  But the header does not show up.
  Here is the code and the result</p>
  
  <pre><code>import Expo from 'expo';
  import React from 'react';
  import { StyleSheet, Text, View, Button } from 'react-native';
  import {StackNavigator} from 'react-navigation';
  
  
  class HomeScreen extends React.Component {
    static navigationOptions = {
      title: 'Welcome',
    }
  
    render() {
      const {navigate} = this.props.navigation;
      return (
        &lt;View style={styles.container}&gt;
          &lt;Text&gt;Open up main.js to start working on your app!&lt;/Text&gt;
          &lt;Button onPress={()=&gt;navigate('Chat',{user:'Lucy'})} title = 'Chat with Lucy'&gt;&lt;/Button&gt;
        &lt;/View&gt;
      );
    }
  }
  class ChatScreen extends React.Component {
    // Nav options can be defined as a function of the screen's props:
    static navigationOptions = ({ navigation }) =&gt; ({
      title: `Chat with ${navigation.state.params.user}`,
    });
    render() {
      // The screen's current route is passed in to `props.navigation.state`:
      const { params } = this.props.navigation.state;
      return (
        &lt;View&gt;
          &lt;Text&gt;Chat with {params.user}&lt;/Text&gt;
        &lt;/View&gt;
      );
    }
  }
  const styles = StyleSheet.create({
    container: {
      flex: 1,
      backgroundColor: '#fff',
      alignItems: 'center',
      justifyContent: 'center',
    },
  });
  const SimpleApp = StackNavigator({
    Home: {screen: HomeScreen},
    Chat: {screen: ChatScreen}
  })
  Expo.registerRootComponent(SimpleApp);
  </code></pre>
  
  <p>And here is the screen result when I click on the button
  <a href="https://i.stack.imgur.com/1qLmW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1qLmW.png" alt="enter image description here"></a></p>
  
  <p>Another problem is that if I only use </p>
  
  <pre><code>static navigationOptions = {
      title: 'Chat with Lucy',
    };
  </code></pre>
  
  <p>Then, the "Welcome" is still next to the mark "&lt;", which is different from the tutorial.</p>
  
  <p><a href="https://i.stack.imgur.com/RL0ma.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RL0ma.png" alt="enter image description here"></a></p>