models: Simplify JSON decoder code

mopidy/models.py

@@ -313,13 +313,11 @@ def model_json_decoder(dct):
 
     """
     if '__model__' in dct:
+        # TODO: move models to a global constant once we split this module
         models = {c.__name__: c for c in ImmutableObject.__subclasses__()}
         model_name = dct.pop('__model__')
         if model_name in models:
-            kwargs = {}
-            for key, value in dct.items():
-                kwargs[key] = value
-            return models[model_name](**kwargs)
+            return models[model_name](**dct)
     return dct