Nginx: conditional "expires" directive

[somebi]

I wanted to create if block for expires directive, but it's not allowed
in if context.

Like that:

if($args ~ "no_caching=1") {
  expires      epoch;
}
Is it somehow possible to create this in any other way?

I need nginx to change expire header to epoch when simple $_GET param
no_cache=1 is in present.

For visual checking purposes.

Thanks ;)

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,213841,213841#msg-213841