util.pubsub: For open nodes, default affiliation is "member"

This allows entities without an explicit affiliation to retrieve items,
which is specified by the XEP. Table 6: "Node Access Models" states that
for 'open' nodes, "any entity may retrieve items from the node".

See also discussion at:
 https://mail.jabber.org/pipermail/standards/2018-August/035320.html

util/pubsub.lua

@@ -216,7 +216,7 @@ function service:get_default_affiliation(node, actor, action) -- luacheck: ignor
 		or self.node_defaults.access_model;
 
 	if access_model == "open" then
-		return "none";
+		return "member";
 	elseif access_model == "whitelist" then
 		return "outcast";
 	end