Bug: sync always omits timeline for left rooms

[Benjamin-L]

Steps to reproduce:

• leave a room
• issue a /sync request, either without a since parameter or with a since parameter from before the user left the room

The expected result is that the sync response will contain a timeline field for the left room that either has all events between since and the point where the user left the room, or has "limited": true and all events between timeline.prev_batch and the point where the user left the room. The current result is that the timeline field is missing entirely.

Notes

There's an associated TODO comment in the code here.

The code currently does this for left rooms:

Timeline {
	limited: false,
	prev_batch: Some(next_batch_string.clone()),
	events: Vec::new(),
}

If we set limited: true instead, then this would technically be a spec-compliant response, however this ruma bug means that the timeline field will be omitted from the response anyway.