mondodb: fix 'sum' aggregation operator

commons-datastore/commons-datastore-mongodb/src/main/java/org/opencb/commons/datastore/mongodb/MongoDBDocumentToFacetFieldsConverter.java

@@ -119,7 +119,7 @@ private MongoDBQueryUtils.Accumulator getAccumulator(Document document) {
             }
         }
         throw new IllegalArgumentException("No accumulators found in facet document: " + StringUtils.join(document.keySet(), ",")
-                + " Valid accumulator functions: " + StringUtils.join(Arrays.asList(count, max, min, avg, stdDevPop, stdDevSamp), ","));
+                + " Valid accumulator functions: " + StringUtils.join(Arrays.asList(count, sum, max, min, avg, stdDevPop, stdDevSamp), ","));
     }
 
     @Override