has_one should have automatic validator

[BrucePerens]

Given model A which has_one model B, not nilable; and model B which belongs_to model A, the foreign key is in Model B. Model A does not automatically validate that Model B exists before it is saved, but it should. For context, see https://gitter.im/luckyframework/Lobby?at=5ec87bab2280c80cbfc19651

[BrucePerens]

Also, has_one doesn't get an attribute in the save operation.

[BrucePerens]

This validator needs to run before commit, and after any after-save callbacks have run. In general, management of dependent records happens in after-save callbacks.