[1.6.x] Fixed #11811 -- Data-loss bug in queryset.update.

It's now forbidden to call queryset.update(field=instance) when instance
hasn't been saved to the database ie. instance.pk is None.

Conflicts:
	tests/queries/tests.py

Backport of b4cd8169 from master.

tests/queries/models.py

@@ -17,9 +17,13 @@ class ProxyCategory(DumbCategory):
     class Meta:
         proxy = True
 
+@python_2_unicode_compatible
 class NamedCategory(DumbCategory):
     name = models.CharField(max_length=10)
 
+    def __str__(self):
+        return self.name
+
 @python_2_unicode_compatible
 class Tag(models.Model):
     name = models.CharField(max_length=10)