Rewrite the ``remote_ip`` and ``protocol`` fields.

def _apply_xheaders(self, headers: httputil.HTTPHeaders) -> None:
        """Rewrite the ``remote_ip`` and ``protocol`` fields."""
        # Squid uses X-Forwarded-For, others use X-Real-Ip
        ip = headers.get("X-Forwarded-For", self.remote_ip)
        # Skip trusted downstream hosts in X-Forwarded-For list
        for ip in (cand.strip() for cand in reversed(ip.split(","))):
            if ip not in self.trusted_downstream:
                break
        ip = headers.get("X-Real-Ip", ip)
        if netutil.is_valid_ip(ip):
            self.remote_ip = ip
        # AWS uses X-Forwarded-Proto
        proto_header = headers.get(
            "X-Scheme", headers.get("X-Forwarded-Proto", self.protocol)
        )
        if proto_header:
            # use only the last proto entry if there is more than one
            # TODO: support trusting mutiple layers of proxied protocol
            proto_header = proto_header.split(",")[-1].strip()
        if proto_header in ("http", "https"):
            self.protocol = proto_header