Support for SSL SNI in BackendSet SSL

I want End to End SSL with SSL in BackendSet to an upstream server.

But, SSL SNI is empty in using SSL in BackendSet.

In my case, $ssl_server_name in an upstream nginx server is empty.

I suppose that SSL connection is termination in Front SSL, but the SSL SNI in Front SSL is not maintained to BackendSet SSL.

I want to maintain the SSL SNI, Front to BackendSet.