Ensure that parent relationships exist on one-to-one relationships

I think that the `parent` key on the `reference` may not be setup correctly on one-to-one relationships (there is a test attached highlighting the issue).

*Is this correct, or are one-to-one relationships not supposed to have a parent in their reference?*

Further, I think this causes an addition PUT to the server when saving a mother record with a one-to-one child record embedded (an extra put for the child, although it's saved with the mother).

This is a related issue regarding many-to-many relationships: https://github.com/emberjs/data/pull/633