be_served_from_origin doesn't work if cache key doesn't include origin hostname

[amcinnes]

The be_served_from_origin checks the hostname in the cache key, and assumes this is going to be the origin hostname. If you set cache key to incoming host header, be_served_from_origin will fail.

If there's no possible way to check the actual origin hostname, perhaps we could deprecate be_served_from_origin and replace it with something called e.g. have_cache_key_hostname that more clearly describes what it does.