title: Bug using geometryfunction in OpenLayers 3 clustering
body: <p>I am trying to exclude certain features from being clustered, by using the geometryfunction as described in the API here:</p>
  
  <p><a href="http://openlayers.org/en/latest/apidoc/ol.source.Cluster.html" rel="nofollow">http://openlayers.org/en/latest/apidoc/ol.source.Cluster.html</a></p>
  
  <p>It is recommended to return null, when the feature should not be considered for clustering. I would expect that to mean, that the feature would still be shown, but not included in clusters. What i see is, that the feature is not shown at all. Is this to be expected?</p>
  
  <pre><code>IconLayer.clusterSource = new ol.source.Cluster({
      distance: 40,
      geometryFunction: this.clusterFunction,
      source: mySource,            
  });
  
  private clusterFunction(icon: Icon) {
      return icon.geo.isMoving ? null : icon.getGeometry();
  }
  </code></pre>