Really logging the POST request body (instead of "-") with nginx

I'm trying to log POST body, and add $request_body to the log_format in http clause, but the access_log command just prints "-" as the body after I send POST request using:

curl -d name=xxxx myip/my_location

My log_format (in http clause):

log_format client '$remote_addr - $remote_user $request_time $upstream_response_time '
                  '[$time_local] "$request" $status $body_bytes_sent $request_body "$http_referer" '
                  '"$http_user_agent" "$http_x_forwarded_for"';

My location definition(in server clause):

location = /c.gif {  
  empty_gif;  
  access_log logs/uaa_access.log client;  
}

How can I print the actual POST data from curl?