code: func (cn *ContentNegotiator) Negotiate(req *http.Request, data interface{}) ([]byte, error) {
  	if len(cn.encoderMap) <= 0 {
  		panic("No Encoders present. Please add them using ContentNegotiator.AddEncoder()")
  	}
  	var e = cn.getEncoder(req)
  	cn.ResponseWriter.Header().Set("Content-Type", e.ContentType())
  	return e.Encode(data)
  }
docs: // Negotiate inspects the request for the accept header and
  // encodes the response appropriately.