Fixed #20625 -- Chainable Manager/QuerySet methods.

Additionally this patch solves the orthogonal problem that specialized
`QuerySet` like `ValuesQuerySet` didn't inherit from the current `QuerySet`
type. This wasn't an issue until now because we didn't officially support
custom `QuerySet` but it became necessary with the introduction of this new
feature.

Thanks aaugustin, akaariai, carljm, charettes, mjtamlyn, shaib and timgraham
for the reviews.

tests/custom_managers/tests.py

@@ -11,12 +11,54 @@ class CustomManagerTests(TestCase):
         p1 = Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
         p2 = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False)
 
+        # Test a custom `Manager` method.
         self.assertQuerysetEqual(
             Person.objects.get_fun_people(), [
                 "Bugs Bunny"
             ],
             six.text_type
         )
+
+        # Test that the methods of a custom `QuerySet` are properly
+        # copied onto the default `Manager`.
+        for manager in ['custom_queryset_default_manager',
+                        'custom_queryset_custom_manager']:
+            manager = getattr(Person, manager)
+
+            # Copy public methods.
+            manager.public_method()
+            # Don't copy private methods.
+            with self.assertRaises(AttributeError):
+               manager._private_method()
+            # Copy methods with `manager=True` even if they are private.
+            manager._optin_private_method()
+            # Don't copy methods with `manager=False` even if they are public.
+            with self.assertRaises(AttributeError):
+               manager.optout_public_method()
+
+            # Test that the overriden method is called.
+            queryset = manager.filter()
+            self.assertQuerysetEqual(queryset, ["Bugs Bunny"], six.text_type)
+            self.assertEqual(queryset._filter_CustomQuerySet, True)
+
+            # Test that specialized querysets inherit from our custom queryset.
+            queryset = manager.values_list('first_name', flat=True).filter()
+            self.assertEqual(list(queryset), [six.text_type("Bugs")])
+            self.assertEqual(queryset._filter_CustomQuerySet, True)
+
+        # Test that the custom manager `__init__()` argument has been set.
+        self.assertEqual(Person.custom_queryset_custom_manager.init_arg, 'hello')
+
+        # Test that the custom manager method is only available on the manager.
+        Person.custom_queryset_custom_manager.manager_only()
+        with self.assertRaises(AttributeError):
+            Person.custom_queryset_custom_manager.all().manager_only()
+
+        # Test that the queryset method doesn't override the custom manager method.
+        queryset = Person.custom_queryset_custom_manager.filter()
+        self.assertQuerysetEqual(queryset, ["Bugs Bunny"], six.text_type)
+        self.assertEqual(queryset._filter_CustomManager, True)
+
         # The RelatedManager used on the 'books' descriptor extends the default
         # manager
         self.assertIsInstance(p2.books, PublishedBookManager)