Clarify inclusion of Multicast-Signaling Option for reverse proxy case

A reverse proxy is behaving like an origin server in most aspects. So, it could allow also unicast requests to a particular proxy resource which trigger a group request, and send back multiple responses to the client, even if the client did not include a Multicast-Signaling Option in the request.

• In this case the Proxy uses a default configured timeout for the request, or has some algorithm to determine when the request is to be closed.
• Due to it being a reverse proxy which MAY be application-specific , it will have some application-level knowledge to determine the timeout of the request.
• The issue of multiple responses being sent to the unaware client is not really an issue because only 'aware' clients would access the particular resource of the reverse proxy.
• And besides, we argue that clients must be prepared to handle multiple responses (from a single server) anyhow at their application layer so this will not break the client.

Proposal: allow for a reverse proxy operation without a Multicast-Signaling Option.