Fix if replicas is set to 0, Fixes #1572

Signed-off-by: Rob Kooper <kooper@illinois.edu>

docker/types/services.py

@@ -407,7 +407,7 @@ class ServiceMode(dict):
                 'replicas can only be used for replicated mode'
             )
         self[mode] = {}
-        if replicas:
+        if replicas is not None:
             self[mode]['Replicas'] = replicas
 
     @property

tests/unit/dockertypes_test.py

@@ -305,6 +305,12 @@ class ServiceModeTest(unittest.TestCase):
         assert mode.mode == 'replicated'
         assert mode.replicas == 21
 
+    def test_replicated_replicas_0(self):
+        mode = ServiceMode('replicated', 0)
+        assert mode == {'replicated': {'Replicas': 0}}
+        assert mode.mode == 'replicated'
+        assert mode.replicas == 0
+
     def test_invalid_mode(self):
         with pytest.raises(InvalidArgument):
             ServiceMode('foobar')