This pull request fixes a bug that occurred under the following circumstances:

• When calling manager.on_conflict(conflict_fields, ConflictAction.NOTHING).bulk_insert(...),
• if conflict_fields includes the name of a ForeignKey field,
• if the primary key of that field's related model is a UUIDField.

When attempting to do this, the following would happen:

• bulk_insert would call _rewrite_insert_nothing(...)
• where_clause would include something like foo_id = %s
• _format_field_value("foo") would call prepare_value, passing it an instance of the Foo model
• get_db_prep_save is called on the ForeignKey field
• get_db_prep_save is called on Foo's primary key (a UUIDField)
• UUIDField.to_python attempts to coerce the instance of the Foo model into a UUID, raising an error.