Fix resetting ContainerSpec properties to None

Signed-off-by: Viktor Adam <rycus86@gmail.com>

docker/api/service.py

@@ -428,7 +428,8 @@ class ServiceApiMixin(object):
                     if 'ContainerSpec' not in merged:
                         merged['ContainerSpec'] = {}
                     for cs_key, cs_value in override['ContainerSpec'].items():
-                        merged['ContainerSpec'][cs_key] = cs_value
-                else:
+                        if cs_value is not None:
+                            merged['ContainerSpec'][cs_key] = cs_value
+                elif ts_value is not None:
                     merged[ts_key] = ts_value
         return merged

tests/integration/models_services_test.py

@@ -193,15 +193,15 @@ class ServiceTest(unittest.TestCase):
             tasks = service.tasks()
         assert len(tasks) == 1
         service.update(
-            # create argument
-            name=service.name,
             mode=docker.types.ServiceMode('replicated', replicas=2),
-            # ContainerSpec argument
-            command="sleep 600"
         )
         while len(tasks) == 1:
             tasks = service.tasks()
         assert len(tasks) >= 2
+        # check that the container spec is not overridden with None
+        service.reload()
+        spec = service.attrs['Spec']['TaskTemplate']['ContainerSpec']
+        assert spec.get('Command') == ['sleep', '300']
 
     @helpers.requires_api_version('1.25')
     def test_restart_service(self):