\RestfulFormatterHalJson::moveReferencesToEmbeds only works on multiple value fields

\RestfulFormatterHalJson::moveReferencesToEmbeds does not work with a field that is configured as being single-value (e.g. taxonomy term reference). (Possibly related: #282)
``RestfulViewEntityTestCase`` is becoming a beast. Maybe we can move this to own class - ``RestfulHalJsonTestCase``?