Correction of the the unit option for the GeoDistance filter (changed fr...

This change fixes the unit option on the GeoDistance filter.  There was an error in the Elasticsearch documentation stating that the parameter for specifying the distance unit was "distance_unit", however it is actually just "unit".  This has since been corrected (See: https://github.com/elasticsearch/elasticsearch/issues/2771), but NEST was using the incorrect "distance_unit" parameter.  Before this change, specifying the distance unit would cause Elasticsearch to throw a parsing exception.  I've tested and confirmed this change in my application and have updated the unit test accordingly.