Docs lack example of how to include a custom JSONEncoder subclass into serializers.serialize()

[Oroku Saki]

In this section ​http://docs.djangoproject.com/en/dev/topics/serialization/#id2 it describes an example of where you would need a custom encoder class, but gives no example of how to add your class. My guess was using serialize(obj, cls=MyCustomClass), but this raised a "multiple values of 'cls' keyword argument" error. I ended up not needing a custom class, but it might be helpful for others to see it in use.

[Tim Graham]

duplicate of #15091