filter out headers for fastcgi cache

Greetings,

I would like to propose that an additional feature for nginx be 
considered related to fastcgi caching: the ability to specify headers 
that will not be stored in cache, but will be sent to the client when 
the cached version is created (sent to the client responsible for 
creating the cached copy).  If some solution already exists providing 
this functionality, my apologies, I was not able to track one down - 
currently assuming one does not exist.

Here is one scenario where such an option would be useful (I am sure 
there are others):

A typical scenario where fastcgi caching can be employed for performance 
benefits is when the default version of a page is loaded.  By "default", 
I mean the client has no prior session data which might result in unique 
session specific request elements.  In the case of PHP, the presence of 
session data is typically determined by checking for the presence of a 
"PHPSESSID" cookie.  So if this cookie does not exist, then it can be 
assumed there is no session - an optimal time to create a cached version 
of the page in many scenarios.  However, many PHP apps/sites/etc. also 
create a session in the event one does not exist (a behavior I assume is 
not specific to PHP) - meaning the the response typically contains a 
Set-Cookie: PHPSESSID.... header.  Nginx's default behavior is not to 
cache any page with a Set-Cookie header, and that makes sense as a 
default - but lets assume for this example that fastcgi_ignore_headers 
Set-Cookie; is in effect and the cached version of the default version 
of the page gets created.  The problem here is that the cached version 
created also has the Set-Cookie header cached as well - which causes 
problems for obvious reasons (hands out the same session ID/cookie for