bugfix: parsing of team_join payload (#15)

Author: johnkors

source/src/Slackbot.Net.Endpoints/Middlewares/HttpItemsManager.cs

@@ -75,6 +75,8 @@ private static SlackEvent ToEventType(JsonElement eventJson, string raw)
                 return JsonSerializer.Deserialize<MemberJoinedChannelEvent>(json, WebOptions);
             case EventTypes.AppHomeOpened:
                 return JsonSerializer.Deserialize<AppHomeOpenedEvent>(json, WebOptions);
+            case EventTypes.TeamJoin:
+                return JsonSerializer.Deserialize<TeamJoinEvent>(json, WebOptions);
             default:
                 var unknownSlackEvent = JsonSerializer.Deserialize<UnknownSlackEvent>(json, WebOptions);
                 unknownSlackEvent.RawJson = raw;