[commands] Allow Converter instances in Greedy. Fix #1939.

discord/ext/commands/converter.py

@@ -500,8 +500,8 @@ class _Greedy:
             raise TypeError('Greedy[...] only takes a single argument')
         converter = params[0]
 
-        if not inspect.isclass(converter):
-            raise TypeError('Greedy[...] expects a type.')
+        if not inspect.isclass(converter) and not isinstance(converter, Converter):
+            raise TypeError('Greedy[...] expects a type or a Converter instance.')
 
         if converter is str or converter is type(None) or converter is _Greedy:
             raise TypeError('Greedy[%s] is invalid.' % converter.__name__)