models: Shortcut case where copy didn't change anything

We no longer copy in this case and will just give you the same instance back.

mopidy/models.py

@@ -224,6 +224,8 @@ class ImmutableObject(object):
         :type values: dict
         :rtype: new instance of the model being copied
         """
+        if not values:
+            return self
         other = copy.copy(self)
         for key, value in values.items():
             if key not in self._fields:

tests/models/test_models.py

@@ -12,7 +12,7 @@ class GenericCopyTest(unittest.TestCase):
 
     def compare(self, orig, other):
         self.assertEqual(orig, other)
-        self.assertNotEqual(id(orig), id(other))
+        self.assertEqual(id(orig), id(other))
 
     def test_copying_track(self):
         track = Track()