How to extract multiple values of a header for logging

[keeyong]

I have a custom header in http response and it can appear more than once.
Let's say it is called x-extra. 

In my nginx configuration, I want to extract all of its values and log them
together but I found it is always extracting the very first one. Is it
possible to do this somehow? 

Thanks in advance!
-K

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,254961,254961#msg-254961