Fixed separator in $sent_http_cache_control.

In case multiple "Cache-Control" headers are sent to a client,
multiple values in $sent_http_cache_control were incorrectly
split by a semicolon.  Now they are split by a comma.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5084 73f98a42-aea0-e011-b76d-00259023448c