Properly support @commands.check() decorator

Author: eunwoo1104

discord_slash/model.py

@@ -34,6 +34,8 @@ def __init__(self, name, cmd):  # Let's reuse old command formatting.
         # Since this isn't inherited from `discord.ext.commands.Command`, discord.py's check decorator will
         # add checks at this var.
         self.__commands_checks__ = []
+        if hasattr(self.func, '__commands_checks__'):
+            self.__commands_checks__ = self.func.__commands_checks__
 
     async def invoke(self, *args, **kwargs):
         """