fix dir json output

ml/thing.py

@@ -513,9 +513,13 @@ def to_dir_json(self):
             self.dir_json["attributes"]["thingStructure"]["links"].append(role_entry)
 
         for key in self.features.keys():
+            feature_target = {
+                "class": self.features[key].to_json()["class"],
+                "identifier": self.features[key].to_json()["identifier"]
+            }
             feature_entry = {
                 "association": "features",
-                "target": self.features[key].to_json()
+                "target": feature_target
             }
             # if the feature has targets, like ml40::Composite
             if hasattr(self.features[key], 'targets'):
@@ -576,9 +580,13 @@ def to_subthing_dir_json(self):
             }
             json_out["links"].append(role_entry)
         for key in self.features.keys():
+            feature_target = {
+                "class": self.features[key].to_json()["class"],
+                "identifier": self.features[key].to_json()["identifier"]
+            }
             feature_entry = {
                 "association": "features",
-                "target": self.features[key].to_json()
+                "target": feature_target
             }
             json_out["links"].append(feature_entry)
         return json_out