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"?