Fixed #21247 -- Made method_decorator play nicely with descriptors

When a method decorator was used in conjunction with a decorator
implemented as a descriptor, method_decorator did not correctly respect
the method binding.

Thanks for Graham Dumpleton for the report and initial patch.

tests/decorators/tests.py

@@ -232,9 +232,45 @@ class MethodDecoratorTests(TestCase):
             def method(self):
                 return True
 
-        # t = Test()
         self.assertEqual(Test().method(), False)
 
+    def test_descriptors(self):
+
+        def original_dec(wrapped):
+            def _wrapped(arg):
+                return wrapped(arg)
+
+            return _wrapped
+
+        method_dec = method_decorator(original_dec)
+
+        class bound_wrapper(object):
+            def __init__(self, wrapped):
+                self.wrapped = wrapped
+                self.__name__ = wrapped.__name__
+
+            def __call__(self, arg):
+                return self.wrapped(arg)
+
+            def __get__(self, instance, owner):
+                return self
+
+        class descriptor_wrapper(object):
+            def __init__(self, wrapped):
+                self.wrapped = wrapped
+                self.__name__ = wrapped.__name__
+
+            def __get__(self, instance, owner):
+                return bound_wrapper(self.wrapped.__get__(instance, owner))
+
+        class Test(object):
+            @method_dec
+            @descriptor_wrapper
+            def method(self, arg):
+                return arg
+
+        self.assertEqual(Test().method(1), 1)
+
 
 class XFrameOptionsDecoratorsTests(TestCase):
     """