Add support for OneToOne

Hi,

As said on IRC with ocra, I've stumbled into this use case today: I have a User entity with a OneToOne association to a credit card. The user may be created without Credit Card, so I need to be able to POST a new card to an endpoint like "/users/4/card" (because the user can only have one and only one card).

This was not possible because the router extracted data from the entity, but was null

ping @Ocramius @danizord