Add test for when the serializer returns bytes.

ipywidgets/widgets/tests/test_set_state.py

@@ -41,24 +41,24 @@ class DataInstance():
     def __init__(self, data=None):
         self.data = data
 
-def serializer(instance, widget):
+def mview_serializer(instance, widget):
     return { 'data': memoryview(instance.data) if instance.data else None }
 
+def bytes_serializer(instance, widget):
+    return { 'data': memoryview(instance.data).tobytes() if instance.data else None }
+
 def deserializer(json_data, widget):
-    return DataInstance( json_data['data'].tobytes() if json_data else None )
+    return DataInstance( memoryview(json_data['data']).tobytes() if json_data else None )
 
 class DataWidget(SimpleWidget):
-    d = Instance(DataInstance).tag(sync=True, to_json=serializer, from_json=deserializer)
-
-
+    d = Instance(DataInstance).tag(sync=True, to_json=mview_serializer, from_json=deserializer)
 
 # A widget that has a buffer that might be changed on reception:
 def truncate_deserializer(json_data, widget):
     return DataInstance( json_data['data'][:20].tobytes() if json_data else None )
 
 class TruncateDataWidget(SimpleWidget):
-    d = Instance(DataInstance).tag(sync=True, to_json=serializer, from_json=truncate_deserializer)
-
+    d = Instance(DataInstance).tag(sync=True, to_json=bytes_serializer, from_json=truncate_deserializer)
 
 
 #