Improved zoomToShowLayer with callback check
Improved zoomToShowLayer with callback check

Following issue #503, slightly improved `zoomToShowLayer` to check that a proper callback function is passed. Also slightly simplified the event binding on `parent.spiderfy()` (using directly `this.once` instead of manually unbinding it).
frenomus/Leaflet.markercluster,nerik/Leaflet.markercluster,Leaflet/Leaflet.markercluster,ghybs/Leaflet.markercluster,604557209/byzgf,ghybs/Leaflet.markercluster,frenomus/Leaflet.markercluster,Leaflet/Leaflet.markercluster,nerik/Leaflet.markercluster,zverev/Leaflet.markercluster,zverev/Leaflet.markercluster,604557209/byzgf