Otherwise we cannot be sure if the resource we are looking at is a
proxy. Looking at the request as we did in DAVPrincipal is wrong, as one
request can result in several resources to be examined, only some of
which may be proxies.
BTW looking at the regression test changes, I'm not sure having
calendar-proxy-read-for on proxy principals is correct, but caldav-proxy
seems to have nothing to say on that so we leave it for now.
Some clients want to read the group principal before they're happy
to access the group's calendar collections. This grants them the
DAV::read privilege so they can do that.
Also see: https://bugzilla.mozilla.org/show_bug.cgi?id=610087
