Refs r16873 and #16026 -- Missed adding the fixture.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16882 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/modeltests/fixtures/fixtures/invalid.json

+[
+    {
+        "pk": "1",
+        "model": "fixtures.article",
+        "fields": {
+            "headline": null,
+            "pub_date": "2006-06-16 13:00:00"
+        }
+    }
+]