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/basic/tests.py

@@ -6,6 +6,7 @@ import threading
 from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
 from django.db import connections, DEFAULT_DB_ALIAS
 from django.db.models.fields import Field, FieldDoesNotExist
+from django.db.models.manager import BaseManager
 from django.db.models.query import QuerySet, EmptyQuerySet, ValuesListQuerySet, MAX_GET_RESULTS
 from django.test import TestCase, TransactionTestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
@@ -734,3 +735,57 @@ class ConcurrentSaveTests(TransactionTestCase):
         t.join()
         a.save()
         self.assertEqual(Article.objects.get(pk=a.pk).headline, 'foo')
+
+
+class ManagerTest(TestCase):
+    QUERYSET_PROXY_METHODS = [
+        'none',
+        'count',
+        'dates',
+        'datetimes',
+        'distinct',
+        'extra',
+        'get',
+        'get_or_create',
+        'update_or_create',
+        'create',
+        'bulk_create',
+        'filter',
+        'aggregate',
+        'annotate',
+        'complex_filter',
+        'exclude',
+        'in_bulk',
+        'iterator',
+        'earliest',
+        'latest',
+        'first',
+        'last',
+        'order_by',
+        'select_for_update',
+        'select_related',
+        'prefetch_related',
+        'values',
+        'values_list',
+        'update',
+        'reverse',
+        'defer',
+        'only',
+        'using',
+        'exists',
+        '_update',
+    ]
+
+    def test_manager_methods(self):
+        """
+        This test ensures that the correct set of methods from `QuerySet`
+        are copied onto `Manager`.
+
+        It's particularly useful to prevent accidentally leaking new methods
+        into `Manager`. New `QuerySet` methods that should also be copied onto
+        `Manager` will need to be added to `ManagerTest.QUERYSET_PROXY_METHODS`.
+        """
+        self.assertEqual(
+            sorted(BaseManager._get_queryset_methods(QuerySet).keys()),
+            sorted(self.QUERYSET_PROXY_METHODS),
+        )