Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

tests/defer_regress/tests.py

@@ -7,6 +7,7 @@ from django.contrib.sessions.backends.db import SessionStore
 from django.db.models import Count
 from django.db.models.loading import cache
 from django.test import TestCase
+from django.test.utils import override_settings
 
 from .models import (
     ResolveThis, Item, RelatedItem, Child, Leaf, Proxy, SimpleItem, Feature,
@@ -83,24 +84,6 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(results[0].child.name, "c1")
         self.assertEqual(results[0].second_child.name, "c2")
 
-        # Test for #12163 - Pickling error saving session with unsaved model
-        # instances.
-        SESSION_KEY = '2b1189a188b44ad18c35e1baac6ceead'
-
-        item = Item()
-        item._deferred = False
-        s = SessionStore(SESSION_KEY)
-        s.clear()
-        s["item"] = item
-        s.save()
-
-        s = SessionStore(SESSION_KEY)
-        s.modified = True
-        s.save()
-
-        i2 = s["item"]
-        self.assertFalse(i2._deferred)
-
         # Regression for #16409 - make sure defer() and only() work with annotate()
         self.assertIsInstance(
             list(SimpleItem.objects.annotate(Count('feature')).defer('name')),
@@ -147,6 +130,27 @@ class DeferRegressionTest(TestCase):
                 cache.get_app("defer_regress"), include_deferred=True))
         )
 
+    @override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer')
+    def test_ticket_12163(self):
+        # Test for #12163 - Pickling error saving session with unsaved model
+        # instances.
+        SESSION_KEY = '2b1189a188b44ad18c35e1baac6ceead'
+
+        item = Item()
+        item._deferred = False
+        s = SessionStore(SESSION_KEY)
+        s.clear()
+        s["item"] = item
+        s.save()
+
+        s = SessionStore(SESSION_KEY)
+        s.modified = True
+        s.save()
+
+        i2 = s["item"]
+        self.assertFalse(i2._deferred)
+
+    def test_ticket_16409(self):
         # Regression for #16409 - make sure defer() and only() work with annotate()
         self.assertIsInstance(
             list(SimpleItem.objects.annotate(Count('feature')).defer('name')),