FIX: prevents a circular json error in tests (#14738)

The flow goes from:

- getting current user object
- creating a POJO using some of the current user keys
- passing this POJO around, which end up being used in message bus
- the processing fn associated ens up doing User.create on this object will both create a User object, but also inject store in it, store is holding a reference to currentUser Object and...

BOOM, we have an object holding a reference to the same object, which JSON.stringify used in prepareBody of pretender doesn't like.

app/assets/javascripts/discourse/tests/helpers/presence-pretender.js

@@ -48,7 +48,7 @@ export function joinChannel(name, user) {
     publishToMessageBus(
       `/presence${name}`,
       {
-        entering_users: [user],
+        entering_users: [Object.assign({}, user)],
       },
       0,
       channel.last_message_id