Returns valid map property (_zoom). Fixes #65
Returns valid map property (_zoom). Fixes #65

- When 'retainZoomLevel' is set to true, _getZoomLevel returns zoom of map.
- Whereas there is no such property as 'zoom' for map and hence undefined is returned.
- Correct property is '_zoom' which should be returned to make zoom work fine when retainZoomLevel is set to true.
- Refer the Leaflet source code (getZoom function).

smeijer/L.GeoSearch,smeijer/L.GeoSearch,maettyhawk/leaflet-geosearch,smeijer/leaflet-geosearch,smeijer/leaflet-geosearch,ville6000/leaflet-geosearch