Make nested attributes act as create_or_update

Since :without_protection has been removed from Rails in an effort to move security away from the model, we should allow creation of nested attributes, both when an ID is omitted and when it is present.
allow override of uuid_generate_v4() default by passing default: nil

without this, it's not possible to use UUID primary keys without uuid-ossp installed and activated
Merge remote-tracking branch 'origin/fix-pg-uuid-default'
Merge remote-tracking branch 'origin/allow-nested-ids'

Conflicts:
	activerecord/lib/active_record/nested_attributes.rb