public RoutingResultBuilder negotiatedResponseMediaType(MediaType negotiatedResponseMediaType) {
        this.negotiatedResponseMediaType = requireNonNull(negotiatedResponseMediaType,
                                                          "negotiatedResponseMediaType");
        return this;
    }
Sets the negotiated producible {@link MediaType}.
core/src/main/java/com/linecorp/armeria/server/RoutingResultBuilder.java