The QHttp2Stream implementation doesn't provide any handling or
verification of the HTTP/2 spec. The gRPC over HTTP/2 protocol clearly
defines how Response-Headers, Trailers and Trailers-Only have to behave.

Currently we are missing crucial steps of validation:
 - HTTP status to be 200
 - Trailers contain gRPC status
 - Valid gRPC content-type found

Ref: https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#responses