Have node_setattr delete keys when the value to set is None.

Note that this looks at the value directly; when using {'key': None} as
value, it is not None and thus will be set. This thus is NOT a way to
be 100% sure there are no None values.

tests/test_shots.py

@@ -157,7 +157,7 @@ class NodeSetattrTest(unittest.TestCase):
         node = {}
         node_setattr(node, 'a', None)
         node_setattr(node, None, 'b')
-        self.assertEqual({'a': None, None: 'b'}, node)
+        self.assertEqual({None: 'b'}, node)
 
     def test_none_dotted(self):
         from attract.shots import node_setattr
@@ -167,15 +167,16 @@ class NodeSetattrTest(unittest.TestCase):
 
         node = {'b': {}}
         node_setattr(node, 'b.simple', None)
-        self.assertEqual({'b': {'simple': None}}, node)
+        self.assertEqual({'b': {}}, node)
 
         node_setattr(node, 'b.complex', {'yes': None})
-        self.assertEqual({'b': {'simple': None,
-                                'complex': {'yes': None}}}, node)
+        self.assertEqual({'b': {'complex': {'yes': None}}}, node)
+
+        node_setattr(node, 'b.complex.yes', None)
+        self.assertEqual({'b': {'complex': {}}}, node)
 
         node_setattr(node, 'b.complex', {None: 5})
-        self.assertEqual({'b': {'simple': None,
-                                'complex': {None: 5}}}, node)
+        self.assertEqual({'b': {'complex': {None: 5}}}, node)
 
 
 class PatchShotTest(AbstractShotTest):