[1.5.x] Fixed #19652 -- Fixed .none() regression in related fields

The regression was caused by using .none() when querying for related
models, and the origin field's value was None. This resulted in missing
custom related manager subclass as .none() returns plain QuerySet.

This isn't backport from master, in master .none() correctly preserves
the queryset's class.

Patch provided by Simon Charette, with some minor polish by committer.

tests/modeltests/custom_managers/models.py

@@ -63,3 +63,24 @@ class Car(models.Model):
 
     def __str__(self):
         return self.name
+
+
+# Bug #19652
+class ObjectQuerySet(models.query.QuerySet):
+    pass
+
+class ObjectManager(models.Manager):
+    use_for_related_fields = True
+
+    def get_query_set(self):
+        return ObjectQuerySet(self.model, using=self._db)
+
+
+class RelatedObject(models.Model):
+    pass
+
+
+class Object(models.Model):
+    related = models.ForeignKey(RelatedObject, related_name='objs')
+
+    objects = ObjectManager()