title: Weird navigation behaviour between navigators in react-navigation
body: <p>I am having some trouble trying to navigate between Navigators in react-navigation</p>
  
  <p>Here is my Navigator setup</p>
  
  <pre><code>// Root Navigator
  
  const RootNavigator = DrawerNavigator({
    Charities: {
      screen: CharitiesNavigator
    },
    Events: {
      screen: EventsNavigator
    },
    MapView: {
      screen: MapView
    }
  }, {
    drawerOpenRoute: 'DrawerOpen',
    drawerCloseRoute: 'DrawerClose',
    drawerToggleRoute: 'DrawerToggle',
    backBehaviour: 'none',
    contentComponent: ({navigation})=&gt; &lt;Drawer navigation={navigation} /&gt;
  })
  
  // Charities
  
  const CharitiesNavigator = StackNavigator({
    ChairtiesHome: {
      screen: Charities
    },
    CharityDetail: {
      screen: CharityDetail
    }
  }, {
    headerMode: 'screen',
    backBehaviour: 'none',
    cardStyle: {
      backgroundColor: '#F7F9FA'
    }
  })
  
  // Events
  
  const EventsNavigator = StackNavigator({
    EventsHome: {
      screen: Events
    },
    EventDetail: {
      screen: EventDetail
    }
  }, {
    headerMode: 'screen',
    backBehaviour: 'none',
    cardStyle: {
      backgroundColor: '#F7F9FA'
    }
  })
  </code></pre>
  
  <p>If a user is in the EventDetail screen, they can click a link which will take them to the CharityDetail screen for a given charity. However, if the user then wants to go back, the expected behaviour would be to go back to the EventsDetail screen we came from, but if you use either the hardware button on android or <code>goBack(x)</code> (I have tried null and passing a key) you end up in the Charities Home screen instead.</p>
  
  <p>Any help would be greatly appreciated!</p>