RESP3: PING should reply normally in RESP3 Pub/Sub mode.

Because in RESP3 commands can be sent in the Pub/Sub connection without
problems, so it's better if in such mode there is no exception about
PING.

src/server.c

@@ -2924,7 +2924,7 @@ void pingCommand(client *c) {
         return;
     }
 
-    if (c->flags & CLIENT_PUBSUB) {
+    if (c->flags & CLIENT_PUBSUB && c->resp == 2) {
         addReply(c,shared.mbulkhdr[2]);
         addReplyBulkCBuffer(c,"pong",4);
         if (c->argc == 1)