when grpc server response
SETTINGS[0], WINDOW_UPDATE[0], PING[0], HEADERS[1], DATA[1], HEADERS[1] in one tcp packet,

nginx want to send settings ack and call function ngx_http_grpc_send_settings_ack, write some data in ctx->out

when finalize upstream grpc request, because out != NULL, nginx can't set connection to be keepalive and closed connection.

expect connection to be persistent .