title: Nested TabNavigator not passing params to routes
body: <p>I have a TabNavigator nested in an Stack Navigator. In a login page, I want to navigate to the TabNavigator, resetting the stack and passing some params to the TabNavigator.</p>
  
  <p>As described in the docs, I'm doing the followin to reset the stack, passing the <code>user</code> param to the navigator.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>  resetAction = () =&gt; {
      return NavigationActions.reset({
        index:0,
        actions: [NavigationActions.navigate({routeName:'TabNav', params:{user: userId}})]
      })
    }</code></pre>
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  </p>
  
  <p>My problem is that i can't access the params in the TabNavigator routes. I tried using <code>this.props.navigation.state.params</code> or <code>this.props.navigation.params</code> but none seem to work.</p>
  
  <p>My tabNavigator code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>const TabNav = TabNavigator({
      Page1:{screen: Page1},
      Page2: {screen: Page2},
      Page3: {screen: Page3}
  })</code></pre>
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  </p>