Hi, I discovered another solution, how could be resolved this problem and it's based on headers. I think that it's much cleaner solution.
The headers Content-Type and Accept will be checked, if contains application/vnd.api+json value, as is described here http://jsonapi.org/format/#content-negotiation, if yes so the entities will be serialized by json api serializer.
And I don't know if it's implemented now, but in this step would be good to implement 415 Unsupported Media Type and 406 Not Acceptable responses on the base of this rules http://jsonapi.org/format/#content-negotiation-servers. Or to create new ticket.