Returned domain is lowercased. If the host is invalid, the domain will be empty.
Check that the host looks valid and matches a host or host pattern in the given list of ``allowed_hosts``. Any pattern beginning with a period matches a domain and all its subdomains (e.g. ``.example.com`` matches ``example.com`` and any subdomain), ``*`` matches anything, and anything else must match exactly. Note: This function assumes that the given host is lowercased and has already had the port, if any, stripped off. Return ``True`` for a valid host, ``False`` otherwise.