Fixed #470 -- Added support for database defaults on fields.

Special thanks to Hannes Ljungberg for finding multiple implementation
gaps.

Thanks also to Simon Charette, Adam Johnson, and Mariusz Felisiak for
reviews.

tests/basic/tests.py

@@ -20,6 +20,7 @@ from .models import (
     ArticleSelectOnSave,
     ChildPrimaryKeyWithDefault,
     FeaturedArticle,
+    PrimaryKeyWithDbDefault,
     PrimaryKeyWithDefault,
     SelfRef,
 )
@@ -175,6 +176,11 @@ class ModelInstanceCreationTests(TestCase):
         with self.assertNumQueries(1):
             PrimaryKeyWithDefault().save()
 
+    def test_save_primary_with_db_default(self):
+        # An UPDATE attempt is skipped when a primary key has db_default.
+        with self.assertNumQueries(1):
+            PrimaryKeyWithDbDefault().save()
+
     def test_save_parent_primary_with_default(self):
         # An UPDATE attempt is skipped when an inherited primary key has
         # default.