nginx 1.4.1 + 'gzip on' causes download of file instead of displaying it in browser

[gadh]

when i use nginx 1.4.1 + 'gzip on', once in every 2 requests i get the
index.php (which its output is text/html) being downloaded by the browser as
unknown file type instead of displayed in it (the broswer displays the
"download & save" window and writes in this window that its content type is
'application/octet-stream')
after i download it (firefox saves it with file extension 'part') - i see in
the file the response header (200 OK etc.) + body chunked & still gzipped.
when not using gzip - all works fine.
when switching back to nginx 1.2.8 it never happens - with same
configuration / site / request.
i get to the site thru proxy module, using HTTP/1.1.

thanks in advance
Gad

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,239608,239608#msg-239608