flask-restful unable to add headers into response

Question

I have a flask-restful Resource like so

class UserLogin(Resource):
    @classmethod
    def post(cls):
        return {"msg": "test"}, 200, {"Foo": "bar"}

It should be the case that in the headers of my response, 'Foo' is set to 'bar', but I am unable to see it.

I do see the json body {'msg':'test'} in response, however.