Fixed #24607 -- Serialized natural keys in multi-table inheritance models.

Thanks João Paulo Melo de Sampaio for the test.

tests/serializers/models/multi_table.py

@@ -0,0 +1,19 @@
+from django.db import models
+
+
+class ParentManager(models.Manager):
+    def get_by_natural_key(self, parent_data):
+        return self.get(parent_data=parent_data)
+
+
+class Parent(models.Model):
+    parent_data = models.CharField(max_length=30, unique=True)
+
+    objects = ParentManager()
+
+    def natural_key(self):
+        return (self.parent_data,)
+
+
+class Child(Parent):
+    child_data = models.CharField(max_length=30, unique=True)