Joint effort between myself, Josh, Anssi and Shai.
Conflicts:
django/db/models/query.py
tests/model_fields/models.py
Backport of 4755f8fc25331c739a6f932cc8aba0cc9e62e352 from master.
The method is mainly intended for use with UUIDField. For UUIDField we
want to call the field's default even when primary key value is
explicitly set to None to match the behavior of AutoField.
Thanks to Marc Tamlyn and Tim Graham for review.
Backport of 8adc59038cdc6ce4f9170e4de2d716d940e136b3 from master