Query DSL: custom score script, allow to use `_score` as well as `score` as the underlying query score, closes #316.

modules/elasticsearch/src/main/java/org/elasticsearch/index/query/xcontent/CustomScoreQueryParser.java

@@ -120,6 +120,7 @@ public class CustomScoreQueryParser extends AbstractIndexComponent implements XC
 
         @Override public float score(int docId, float subQueryScore) {
             vars.put("score", subQueryScore);
+            vars.put("_score", subQueryScore);
             return ((Number) scriptFieldsFunction.execute(docId, vars)).floatValue();
         }