Re-enable disableClusteringAtZoom / maxZoom in singleMarkerMode

When you set disableClusteringAtZoom=14, clustering stops and you see individual markers when the user is at zoom 14+.

Currently if you have singleMarkerMode=true, it replaces the marker's icon with a cluster div, which makes it impossible for the original marker to be shown. At zoom 14+ you see each marker as a size-1 cluster.

I needed these options to work together, so I patched it like this. Will gladly amend to fit proper style, especially for carrying this_map and this._maxZoom into the zoomend function.