support ctx->set_http_code() in process();

[holmes1412]

Usage:

class ExampleServiceImpl : public Example::Service                                 
{                                                                                  
public:                                                                            
    void Echo(EchoRequest *req, EchoResponse *resp, RPCContext *ctx) override   
    {                                                                              
        if (req->name() != "workflow")
            ctx->set_http_code("404");
        else
            resp->set_message("Hi back"); 
    }                                                                              
};