fix: Call GuildAvailableAsync for dispatch(GUILD_CREATE) case (#1473)

* Fix for Issue #1471

This change will allow `GuildAvailable` to fire when the client joins a new guild, as well as properly update `IsConnected`.

* Removed unnecessary statement;

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -640,6 +640,7 @@ namespace Discord.WebSocket
                                         if (guild != null)
                                         {
                                             await TimedInvokeAsync(_joinedGuildEvent, nameof(JoinedGuild), guild).ConfigureAwait(false);
+                                            await GuildAvailableAsync(guild).ConfigureAwait(false);
                                         }
                                         else
                                         {