Realm-cocoa: Allow "create or update" for models without primary keys

[jpsim]

If a root model has no primary key, but you want to automatically "create or update" some of its linked objects, there's no way to do that at the moment. Allowing calling "create or update" for models without primary keys could be a way for us to support this very valid use case.

Originally requested in #2590.

[yasirmturk]

Great ..thanks

[bendikv]

Nothing new or at least estimation?

[jpsim]

@bendikv this is not a high priority for us to work on at the moment. See issues labeled P1/P2/In Progress/Review for an up-to-date list of our priorities.