util.sasl: Preserve 'userdata' field between clones

The :clean_clone() method is designed to provide a new cloned SASL handler,
to be used when starting a fresh SASL negotiation on an existing connection.

The userdata field is currently populated by mod_saslauth with the "read-only"
information that the channel binding methods need to do their stuff.

When :clean_clone() does not preserve this, it causes tracebacks in the cb
profile handlers due to the property being nil.

This does mean that SASL handlers should now not be reused (even when cloned)
across different connections, if they ever could.

util/sasl.lua

@@ -67,7 +67,7 @@ local function registerMechanism(name, backends, f, cb_backends)
 end
 
 -- create a new SASL object which can be used to authenticate clients
-local function new(realm, profile)
+local function new(realm, profile, userdata)
 	local mechanisms = profile.mechanisms;
 	if not mechanisms then
 		mechanisms = {};
@@ -80,7 +80,12 @@ local function new(realm, profile)
 		end
 		profile.mechanisms = mechanisms;
 	end
-	return setmetatable({ profile = profile, realm = realm, mechs = mechanisms }, method);
+	return setmetatable({
+		profile = profile,
+		realm = realm,
+		mechs = mechanisms,
+		userdata = userdata
+	}, method);
 end
 
 -- add a channel binding handler
@@ -94,7 +99,7 @@ end
 
 -- get a fresh clone with the same realm and profile
 function method:clean_clone()
-	return new(self.realm, self.profile)
+	return new(self.realm, self.profile, self.userdata)
 end
 
 -- get a list of possible SASL mechanisms to use