models: Make .copy(foo=None) null out field.

mopidy/models.py

@@ -69,10 +69,14 @@ class ImmutableObject(object):
         data = {}
         for key in self.__dict__.keys():
             public_key = key.lstrip('_')
-            data[public_key] = values.pop(public_key, self.__dict__[key])
+            value = values.pop(public_key, self.__dict__[key])
+            if value is not None:
+                data[public_key] = value
         for key in values.keys():
             if hasattr(self, key):
-                data[key] = values.pop(key)
+                value = values.pop(key)
+                if value is not None:
+                    data[key] = value
         if values:
             raise TypeError(
                 'copy() got an unexpected keyword argument "%s"' % key)

tests/test_models.py

@@ -53,6 +53,10 @@ class GenericCopyTest(unittest.TestCase):
         test = lambda: Track().copy(invalid_key=True)
         self.assertRaises(TypeError, test)
 
+    def test_copying_track_to_remove(self):
+        track = Track(name='foo').copy(name=None)
+        self.assertEquals(track.__dict__, Track().__dict__)
+
 
 class RefTest(unittest.TestCase):
     def test_uri(self):