title: React Navigation header not hiding
body: <p>I have a react project and I have two navigators, I'm trying to remove the navigation bar, I tried two ways but no success.
  This is my App.js</p>
  <pre><code>const switchNavigator = createStackNavigator({
    loginFlow: createStackNavigator({
      Signin: {
        screen: SigninScreen,
        navigationOptions: {
          headerShown: 'false'
        }
      },
      Signup: {
        screen: SignupScreen,
      },
    },
    ),
    mainGrid: createStackNavigator({
      Account: AccountScreen,
      Bath: BathScreen,
      Eco: EcoBath,
      Electricity: ElectricityConsScreen,
      Water: WaterConsumptionScreen,
      Help: HelpScreen,
      Parents: ParentsControlScreen
    })
  });
  </code></pre>
  <p>As you can see I tried o hide using the headerShown: 'false' I tried to change to 'hide' and doesn't seems to work. I tried to add separately like:</p>
  <pre><code>SigninScreen.navigationOptions = () =&gt; {
      return{ 
          header: () =&gt; false
      };
  };
  </code></pre>
  <p>But didn't worked too.
  I tried to remove the arrow function and set like header: null and no success too.
  What can I do about this?</p>