Point entity.collection to entity's original collection

[bergie]

In Backbone, Models keep a reference to their collection via model.collection property. This also happens with VIE, but since entities often belong to more than one collection, this is less useful.

For consistency, the model.collection property should always point to the originating collection for the entity, i.e the set of enhancements returned by StanbolService, or the rel parsed by RdfaService.