proxy_cache only if custom header is set by upstream

[ThomasLohner]

Yeah! Thanks, maybe you should add to the wiki, that "if" is executed
before proxy_pass ;-)

Anyways, thank you very much for this helpful answer, it works this
way.

obviously the map has to be the other way around, so if anyone finds
this.. the correct way for caching only if upstream sets a custom header
is:

map $upstream_http_myheader $nocache {
default 1;
1 0;
}

proxy_no_cache $nocache;

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,226461,226467#msg-226467