* FEATURE: Cache CORS preflight requests for 2h
Browsers will cache this for 5 seconds by default. If using MessageBus
in a different domain, Discourse will issue a new long polling, by
default, every 30s or so. This means we would be issuing a new preflight
request **every time**. This can be incredibly wasteful, so let's cache
the authorization in the client for 2h, which is the maximum Chromium
allows us as of today.
* fix tests