title: ClusterManager.clearItems() does not clear items
body: <p>This is a follow-up to my previous question: <a href="https://stackoverflow.com/questions/35022080/clustermanager-isnt-redrawing-markers">ClusterManager isn&#39;t redrawing Markers</a></p>
  
  <p>I've done some investigation and found something interesting. <code>ClusterManager.clearItems()</code> does not always clear items.</p>
  
  <p>After adding one marker to the <code>ClusterManager</code> I run this block:</p>
  
  <pre><code>mClusterManager.clearItems();
  Log.d(TAG, "Marker list is empty: " + String.valueOf(mClusterManager.getMarkerCollection().getMarkers().isEmpty()));
  for (Marker m : mClusterManager.getMarkerCollection().getMarkers()) {
      Log.d(TAG, m.getId() + "| "  + m.getPosition());
  }
  </code></pre>
  
  <p>Which provides the following output:</p>
  
  <pre><code>02-03 12:30:52.953 14328-14328/com.app.mobile D/app:MapsActivity: Marker list is empty: false
  02-03 12:30:52.954 14328-14328/com.app.mobile D/app:MapsActivity: m7| lat/lng: (33.2980945,-111.953964)
  </code></pre>
  
  <p>It is never empty in spite of clear being called. What's going on here?</p>
  
  <p>If it never empties, is adding an updated <code>m7 Marker</code> to the <code>ClusterManager</code> ignored?</p>
  
  <p><a href="https://github.com/googlemaps/android-maps-utils/blob/master/library/src/com/google/maps/android/clustering/algo/NonHierarchicalDistanceBasedAlgorithm.java" rel="nofollow noreferrer">Link to Google's implementation of <code>clearItems()</code></a></p>