Replaced bounds reset method extension by cluster private method
Replaced bounds reset method extension by cluster private method

Following danzel's comment https://github.com/Leaflet/Leaflet.markercluster/pull/584#issuecomment-149748066, replaced the L.LatLngBounds class extension (method reset) by a private method _resetBounds within L.MarkerCluster.

nerik/Leaflet.markercluster,604557209/byzgf,604557209/byzgf,frenomus/Leaflet.markercluster,ghybs/Leaflet.markercluster,frenomus/Leaflet.markercluster,Leaflet/Leaflet.markercluster,ghybs/Leaflet.markercluster,nerik/Leaflet.markercluster,Leaflet/Leaflet.markercluster