net.server: Don't call shutdown method on socket if it doesn't have one, eg. for UDP

net/server.lua

@@ -333,7 +333,7 @@ wrapconnection = function( server, listeners, socket, ip, serverport, clientport
                 send( socket, table_concat( bufferqueue, "", 1, bufferqueuelen ), 1, bufferlen )    -- forced send
             end
         end
-        shutdown( socket )
+        _ = shutdown and shutdown( socket )
         socket:close( )
         _sendlistlen = removesocket( _sendlist, socket, _sendlistlen )
         _socketlist[ socket ] = nil