Support PING while subscribing (RESP2) (#1027)

* Handle PING during pubsub in RESP2

* Rename invalid callback list

Some commands are valid to send during a subscribe in RESP2, and
most in RESP3. Renaming the callback list from `invalid` to `replies`
to detail this fact.

* Fix review comment

1 files changed, 1 insertions, 1 deletions

diff --git a/async.h b/async.h
index b1d2cb2..4c65203 100644
--- a/async.h
+++ b/async.h
@@ -102,7 +102,7 @@ typedef struct redisAsyncContext {
 
     /* Subscription callbacks */
     struct {
-        redisCallbackList invalid;
+        redisCallbackList replies;
         struct dict *channels;
         struct dict *patterns;
     } sub;