Fix GeoHash toString conversion

[sothawo]

In the org.springframework.data.elasticsearch.utils.geohash.Geohash#toLatLon() method the code currently uses

String.format("%f,%f", point.getLat(), point.getLon());

to convert a geopoint to a String containing lat,lon values. This produces a wrong string when running in a Locale where the decimal separator is a comma (germany for example). Need to be fixed to use a defined Locale in formatting