Creating a resource POST with reserved Json-LD key @reverse might fail

From nexus-kg created by [umbreak](https://github.com/umbreak) : BlueBrain/nexus-kg#411

When a payload is posted with the special key `@resverse` on the @context object and without an `@id`, the `graph.primaryNode` call might detect a "wrong" primaryNode.

Note that the logic in `graph.primaryNode` itself is not wrong, because if when designing the the resource one decide to add the `@reverse` key on a property embedded to the "primary" one, it is effectively saying that the primaryNode becomes that property.

I is an edge case, but we have to find out what to do with it. For schemas posted against the platform without `@id`, is not an edge case.

This was already discussed in #405