MSC3069: Allow guests to use /account/whoami (#3069)

* Allow guests to use /account/whoami

* speling

Author: turt2live

proposals/3069-guests-whoami.md

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+# MSC3069: Allow guests to use /account/whoami
+
+Currently the [/account/whoami](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-account-whoami)
+endpoint does not mention anything about guests, which is a bit of an oversight. The implementation
+of the endpoint got created such that guest access was declined.
+
+## Proposal
+
+Guests are allowed to use `/account/whoami`. When a guest makes a request, the response will have
+an added `is_guest: true` field - this field is optional (default `false`) otherwise.
+
+## Potential issues
+
+None forseen. This corrects a mistake.
+
+## Alternatives
+
+None relevant.
+
+## Security considerations
+
+Guests will be able to know their user ID, as they would when they registered in the first place.
+
+## Unstable prefix
+
+While this MSC is not in a stable version of the specification, implementations should use
+`org.matrix.msc3069.is_guest` in place of `is_guest`. Callers should note that they might see
+`M_GUEST_ACCESS_FORBIDDEN` errors if the server is not implementing this MSC.