[commands] default Bot.owner_ids to a set

This appears to be a typo, as everywhere else, owner_ids is set to a set.

discord/ext/commands/bot.py

@@ -108,7 +108,7 @@ class BotBase(GroupMixin):
         self._help_command = None
         self.description = inspect.cleandoc(description) if description else ''
         self.owner_id = options.get('owner_id')
-        self.owner_ids = options.get('owner_ids', {})
+        self.owner_ids = options.get('owner_ids', set())
 
         if self.owner_id and self.owner_ids:
             raise TypeError('Both owner_id and owner_ids are set.')