title: BottomNavigationView not show icon
body: <p>I set an icon for BottomNavigationView but it not show .
  My code:</p>
  
  <pre><code>&lt;android.support.design.widget.BottomNavigationView
      android:id="@+id/navigation"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_gravity="center"
      android:background="?android:attr/windowBackground"
      app:menu="@menu/navigation" /&gt;
  </code></pre>
  
  <p>Menu :
      </p>
  
  <pre><code>&lt;item
      android:id="@+id/navigation_home"
      android:icon="@drawable/ic_tab_bhxh"
      android:title="@string/main_tab_social_bhxh"
      app:showAsAction="ifRoom" /&gt;
  
  &lt;item
      android:id="@+id/navigation_dashboard"
      android:icon="@drawable/ic_tab_bhxh"
      android:title="@string/main_tab_social_bhyt"
      app:showAsAction="ifRoom" /&gt;
  
  &lt;item
      android:id="@+id/navigation_notifications"
      android:icon="@drawable/ic_tab_bhxh"
      android:title="@string/main_tab_social_news"
      app:showAsAction="ifRoom" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>and my program :</p>
  
  <p>If you have any suggest , please tell my know .</p>