Need to support SNI

[xiaoshimimi]

While I'm using https-proxy-agent to send request, if the request option set servername (which might not the same as host) https-proxy-agent will ignore my servername, this will cause the certificate error.

      if (opts.secureEndpoint) {
        // since the proxy is connecting to an SSL server, we have
        // to upgrade this socket connection to an SSL connection
        debug('upgrading proxy-connected socket to TLS connection: %o', opts.host);
        opts.socket = socket;
        **opts.servername = opts.host;**
        opts.host = null;
        opts.hostname = null;
        opts.port = null;
        sock = tls.connect(opts);
      }

Could you please change this part to:
opts.servername = opts.servername?opts.servername:opts.host;
so that it can support SNI?