Feature suggestion from customer: validate hash keys

[SimonWoolf]

trafficstars

cf https://www.ably.io/documentation/general/authentication#token-authentication

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[mattheworiordan]

@SimonWoolf did you close this because of my feedback about issues relating to new attributes needing to be explicitly added? If so, I expect there are places we could check keys and places where it would not be appropriate.

[SimonWoolf]

opened it to bring to your attention, but then you appeared in the chat, so shrug :)

[mattheworiordan]

but then you appeared in the chat, so shrug :)

Ok, sorry! Well let's leave open and I can review during the next burst of work.