fix to_json() bug

ml/thing.py

@@ -525,7 +525,7 @@ def to_json(self):
             self.dt_json["attributes"]["features"] = list()
         for key in self.roles.keys():
             self.dt_json["attributes"]["roles"].append(self.roles[key].to_json())
-        for key in self.features:
+        for key in self.features.keys():
             self.dt_json["attributes"]["features"].append(self.features[key].to_json())
         return self.dt_json