Clear out _needsRemoving when clearLayers is called while we aren't on the map. Otherwise we'll try remove them when we are added, which breaks things.
Clear out _needsRemoving when clearLayers is called while we aren't on the map. Otherwise we'll try remove them when we are added, which breaks things.

Refs #860

ghybs/Leaflet.markercluster,Leaflet/Leaflet.markercluster,ghybs/Leaflet.markercluster,Leaflet/Leaflet.markercluster