[commands] Case insensitivity uses casefold instead of lower

There seems to be very little performance regression in doing so (+5ns)
so might as well go for the more correct behaviour.

discord/ext/commands/core.py

@@ -105,22 +105,22 @@ def _convert_to_bool(argument):
 
 class _CaseInsensitiveDict(dict):
     def __contains__(self, k):
-        return super().__contains__(k.lower())
+        return super().__contains__(k.casefold())
 
     def __delitem__(self, k):
-        return super().__delitem__(k.lower())
+        return super().__delitem__(k.casefold())
 
     def __getitem__(self, k):
-        return super().__getitem__(k.lower())
+        return super().__getitem__(k.casefold())
 
     def get(self, k, default=None):
-        return super().get(k.lower(), default)
+        return super().get(k.casefold(), default)
 
     def pop(self, k, default=None):
-        return super().pop(k.lower(), default)
+        return super().pop(k.casefold(), default)
 
     def __setitem__(self, k, v):
-        super().__setitem__(k.lower(), v)
+        super().__setitem__(k.casefold(), v)
 
 class Command(_BaseCommand):
     r"""A class that implements the protocol for a bot text command.