feat(map): allow custom iconView on GMSMarker subclasses (#506)

Previously, when subclassing GMSMarker with a custom iconView, the iconView would not display on the map. This update enables custom iconViews to render correctly when using GMSMarker subclasses.

Author: talipboke

Sources/GoogleMapsUtilsObjC/include/GMUDefaultClusterRenderer.m

@@ -284,7 +284,7 @@ - (void)renderCluster:(id<GMUCluster>)cluster animated:(BOOL)animated {
   } else {
     for (id<GMUClusterItem> item in cluster.items) {
       GMSMarker *marker;
-      if ([item class] == [GMSMarker class]) {
+      if ([item isKindOfClass:[GMSMarker class]]) {
         marker = (GMSMarker<GMUClusterItem> *)item;
         marker.map = _mapView;
       } else {