Fixed the post-processing of results

If an unsupervised model does not contain an Output element, then the
JPMML-Evaluator library is likely to return its results
in the form of an unmodifiable java.util.Map:
Collections.unmodifiableMap(null, new Computable(){})

openscoring-service/src/main/java/org/openscoring/service/ModelResource.java

@@ -555,13 +555,16 @@ protected EvaluationResponse evaluate(Evaluator evaluator, EvaluationRequest req
 	}
 
 	static
-	private <V> Map<FieldName, V> replaceNullKey(Map<FieldName, V> result){
+	private <V> Map<FieldName, V> replaceNullKey(Map<FieldName, V> map){
 
-		if(result.containsKey(null)){
+		if(map.containsKey(null)){
+			Map<FieldName, V> result = new LinkedHashMap<>(map);
 			result.put(ModelResource.DEFAULT_NAME, result.remove(null));
+
+			return result;
 		}
 
-		return result;
+		return map;
 	}
 
 	static