Fixed #13252 -- Added ability to serialize with natural primary keys.

Added ``--natural-foreign`` and ``--natural-primary`` options and
deprecated the ``--natural`` option to the ``dumpdata`` management
command.

Added ``use_natural_foreign_keys`` and ``use_natural_primary_keys``
arguments and deprecated the ``use_natural_keys`` argument to
``django.core.serializers.Serializer.serialize()``.

Thanks SmileyChris for the suggestion.

tests/serializers_regress/models.py

@@ -118,6 +118,7 @@ class NaturalKeyAnchor(models.Model):
     objects = NaturalKeyAnchorManager()
 
     data = models.CharField(max_length=100, unique=True)
+    title = models.CharField(max_length=100, null=True)
 
     def natural_key(self):
         return (self.data,)