http gzip compression problem for nginx as reverse proxy

I use nginx as reverse proxy server to add load balance function to our web servers.
I have configured my web servers right, that I can get gized contents directly from our web server.
When I through nginx reverse proxy, I get the ungized contents. I didn't know why this happens. Maybe the nginx didn't pass the http header
Accept-Encoding:gzip to the real web server, or nginx just decompress the gziped content from web server and sent unziped content to
client.

How can I tell the nginx not to do these two weird stuff, and just pass the gized content from the real web server to the client.
I found the ngx_http_gzip_module module, which I think is for nginx as http server not as proxy server.