Serialize PKs using the natural key

[Juarez Bochi]

Django uses natural keys only to serialize FKs and M2M fields. Serializing the PK with the natural key can also be useful.

Assuming the following object:

   {
      "pk": 1, 
      "model": "auth.permission", 
      "fields": {
         "codename": "add_logentry", 
         "name": "Can add log entry", 
         "content_type": [
            "admin", 
            "logentry"
         ]
      }
   }, ...

If we use the command loaddata to import this object to the database, an error will be raised if the permission add_logentry already exist with a different id or if another permission has the id 1. That problem can be avoided by using the natural key instead of the pk id number:

   {
      "pk": [
         "add_logentry", 
         "admin", 
         "logentry"
      ], 
      "model": "auth.permission", 
      "fields": {
         "codename": "add_logentry", 
         "name": "Can add log entry", 
         "content_type": [
            "admin", 
            "logentry"
         ]
      }
   }, ...

[Chris Beaven]

Dupe of #13252