Setting custom headers in the site.ini.append.php file:

[HTTPHeaderSettings]
CustomHeader=enabled
HeaderList[]
HeaderList[]=Cache-Control

Cache-Control[]
Cache-Control[/]=public, max-age=1800, smax-age=1800

Generates a: "Cache-Control: public, max-age=1800, smax-age=1800, must-revalidate, no-cache, private"
header instead of just "Cache-Control: public, max-age=1800, smax-age=1800".

Further Notes:

When viewed under Google Chrome, the headers are displayed as:

Cache-Control:must-revalidate, no-cache, private
Cache-Control:public, max-age=1800, smax-age=1800

The first line appears to originate from PHP's built-in session.cache_limiter