'payload' attribute of 'execute_reply' message is optional according to the spec

[Alex Tugarev]

We had to add an empty 'payload' to 'execute_reply' messages to make use of jupyter_kernel_test, but according the messaging spec it should be optional. Actually I'm not sure which one is right.

cf. https://github.com/jupyter/jupyter_client/blob/master/docs/messaging.rst#execution-results

[Thomas Kluyver]

@minrk should payload still be optional now that they're not-so-imminently deprecated? It seems more consistent that it always be there, even if it's an empty list.

[Min RK]

Since the spec says they are optional, I'd let them be optional in the test, and only check that they are the right type if/when present. Both JupyterLab and current notebook appear to handle undefined payloads.

[Steven Silvester]

Fixed in #58