add the required rel attribute for Siren sub-entities
maintain interface compatibility across all adapters
allow providing an array of values for a siren sub-entity rel.

For Hal and Json Api, if the entity name is an array, we just grab the first
name in the array.
Wrap Siren rel in array without the ternary operator
Merge branch 'apsoto-add_siren_subentity_rel'