[commands] Fix issue with decorator order with checks and cooldowns

Now they're just explicitly copied.

discord/ext/commands/cooldowns.py

@@ -98,6 +98,11 @@ class CooldownMapping:
         self._cache = {}
         self._cooldown = original
 
+    def copy(self):
+        ret = CooldownMapping(self._cooldown)
+        ret._cache = self._cache.copy()
+        return ret
+
     @property
     def valid(self):
         return self._cooldown is not None

discord/ext/commands/core.py

@@ -268,6 +268,10 @@ class Command(_BaseCommand):
         ret = self.__class__(self.callback, **self.__original_kwargs__)
         ret._before_invoke = self._before_invoke
         ret._after_invoke = self._after_invoke
+        if self.checks != ret.checks:
+            ret.checks = self.checks.copy()
+        if self._buckets != ret._buckets:
+            ret._buckets = self._buckets.copy()
         try:
             ret.on_error = self.on_error
         except AttributeError: