Make a to_one JSON API compliant

        :spec:
            jsonapi.org/format/#document-resource-object-relationships
        :param key:
            the string name of the relationship field
        :param val:
            dict containing `rid` & `rtype` keys for the to_one &
            None if the to_one is null
        :return:
            dict as documented in the spec link

def _serialize_to_one(self, key, val, rlink):
        """ Make a to_one JSON API compliant

        :spec:
            jsonapi.org/format/#document-resource-object-relationships
        :param key:
            the string name of the relationship field
        :param val:
            dict containing `rid` & `rtype` keys for the to_one &
            None if the to_one is null
        :return:
            dict as documented in the spec link
        """

        data = None
        if val and val['rid']:
            data = {'id': val['rid'], 'type': val['rtype']}

        return {
            key: {
                'data': data,
                'links': {
                    'related': rlink + '/' + key
                }
            }
        }