def starttls(options = {}, verify = true)
      send_command("STARTTLS") do |resp|
        if resp.kind_of?(TaggedResponse) && resp.name == "OK"
          begin
            # for backward compatibility
            certs = options.to_str
            options = create_ssl_params(certs, verify)
          rescue NoMethodError
          end
          start_tls_session(options)
        end
      end
    end
Sends a STARTTLS command to start TLS session.