Does the nginx support "Header append ..."?

[sunjipeng_neu]

For Apache Httpd the "Header" directive can "append" a value to the
exsisting headers of the same name. 

e.g.

Before:   Vary: Host

Header append Vary User-Agent

After:     Vary: Host, User-Agent


But for Nginx, neither "add_header" nor "more_set_headers" directive can
"append" a value to an exsisting headers.

Especially, the "more_set_headers" will create a duplicate headers of
the same name. i.e.

Vary: Host
Vary: User-Agent

So, What's the reason for it? How should I do for the "append"?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,179060,179060#msg-179060