fix dir json print out problem

ml/thing.py

@@ -515,9 +515,7 @@ def to_dir_json(self):
         for key in self.features.keys():
             feature_entry = {
                 "association": "features",
-                "target": {
-                    "class": self.features[key].class_name,
-                }
+                "target": self.features[key].to_json()
             }
             # if the feature has targets, like ml40::Composite
             if hasattr(self.features[key], 'targets'):