title: React Navigation params doesn't reset
body: <p>I'm having trouble with resetting the navigation params to null in React Native.</p>
  
  <p>MainTab<br>
   -- Home (stack)<br>
   -- Misc (stack)<br>
   -- Tips (stack)  </p>
  
  <p>On the Home tab, I have a button to go to Misc, but I want to route to the Tips tab on route to Misc.<br>
  Routing should look like - (Home -> Tips -> Misc)<br>
  That button returns the following with params -    </p>
  
  <pre class="lang-js prettyprint-override"><code>this.props.navigation.navigate('Tips', {backRoute: 'Home', routeImGoingNext: 'Misc'});
  </code></pre>
  
  <p>When these params are passed, I render a back button and a skip button on the Tips screen's navigation based on the backRoute and routeImGoingNext params that were passed from the button on the Home tab.</p>
  
  <pre class="lang-js prettyprint-override"><code>if(navigation.state.params &amp;&amp; navigation.state.params.backRoute){
    return {
      headerLeft: (&lt;HeaderBackButton onPress={()=&gt;navigation.navigate(navigation.state.params.backRoute)}/&gt; ),
      headerRight: (
        &lt;TouchableOpacity onPress={()=&gt;navigation.navigate(navigation.state.params.routeImGoingnext)}&gt;
          &lt;Text style={{paddingRight: 10}}&gt; Skip &lt;/Text&gt;
        &lt;/TouchableOpacity&gt;
      )
    }
  }
  </code></pre>
  
  <p>My problem occurs when I click the Tips tab after I've already clicked the the button on the Home tab. The params are still set and therefore rendering a back button and skip button, but there shouldn't be those buttons if I click the Tips tab.</p>
  
  <p>Any ideas on how to reset the params when you manually click on the tabs?</p>