请教Nginx 中对 application/json 格式进行 gzip 压缩无效的问题

nginx.conf

gzip on;
gzip_min_length 1k;
gzip_buffers 4 16k;
gzip http_version 1.0;
gzip_comp_level 2;
gzip_types text/plain application/json;
gzip_vary on;
gzip_proxied expired no-cache no-store private auth;

我用的是web.py框架
web.header("Content-Type", "application/json")
web.header("Content-Encoding", "gzip")

curl -I返回的是

HTTP/1.1 200 OK
Server: nginx
Date: Tue, 27 Jan 2015 05:32:34 GMT
Content-Type: application/json
Connection: keep-alive
Content-Encoding: gzip
Set-Cookie: webpy_session_id=0b019c20c14f05cd3db73afe18ca07902928889d; Path=/; httponly