HTTP_ACCEPT headers are not being spoofed

I enabled "Spoof US English Browser" in Torbutton Preferences but I still get following headers

text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8 UTF-8,* gzip, deflate ab,en-us;q=0.7,en;q=0.3

(which are the "correct" one) when accessed https://panopticlick.eff.org/index.php?action=log. "User Agent" is being spoofed as expected.

I have Firefox 4b8 with Torbutton 1.3.1-alpha. It worked fine with Firefox 3.6 & Torbutton 1.2.5.

Trac:
Username: sou-druh