Can't get nginx to reply to ab with compressed content

Can't get nginx to reply to ab with compressed content

ab -n 1 -H 'Accept-Encoding: gzip, deflate' http://mywebsite.com/
This always returns the uncompressed content (I can see that both from its
size and because I'm sniffing the traffic with ngrep).
However, if I request the content with Firefox, the result is compressed.
Content-Encoding: gzip it says. I've tried to specify with ab all request
headers Firefox specifies, as I see them sniffed, but still ab gets it
uncompressed and Firefox gets it compressed.