Add another constructor do RangeFacetMap

The class org.apache.solr.client.solrj.request.json.RangeFacetMap is missing the constructor:

RangeFacetMap(final String fieldName, final String start, final String end, final String gap) {
{{...
}}{{}
}}

The constructor is needed, because something like:

new RangeFacetMap("date", "NOW/YEAR-50YEARS", "NOW/DAY", "+1YEAR");

is perfectly valid