Bug 1103331 — Get room public information if not participating

Interpreted https://bugzilla.mozilla.org/show_bug.cgi?id=1103331 as I could.

I think it differs from what was proposed by @ametaireau, since I don't use the POST with ```action == 'join'```
Why would we want that ? Can you explicit please ?

```
> JSON.stringify({ctime:undefined})
"{}"
```