Correct schema write test (#1416)

<Mock>.called_once() just returns a Mock, so assert <Mock>.called_once()
always passes. We want <Mock>.assert_called_once().

Author: kiendang

graphene_django/tests/test_command.py

@@ -46,7 +46,7 @@ class Query(ObjectType):
     open_mock.assert_called_once()
 
     handle = open_mock()
-    assert handle.write.called_once()
+    handle.write.assert_called_once()
 
     schema_output = handle.write.call_args[0][0]
     assert schema_output == dedent(