[Flow] Unexpected Caching

[Peter R]

Hello,

I got a new problem.
I want to use the headers "Last-Modified" and "If-Modified-Since" to make lists shorter.
If client requesting on a list view. I response the list and "Last-Modified" header.
$this->response->setLastModified($lastModified);
The variable $lastModified (new \DateTime()) is the latest date of a record.

The client sends in the next request "If-Modified-Since" with the value of "Last-Modified".
This should produce an empty page with status code "304 Not Modified", but the output is the same like in the first request.

When I clear the browser cache (Firefox 28), I get "304 Not Modified".
When I don't use "Last-Modified", the output is not cached.

Is this a browser or a Flow problem?
The iOS App who is requesting the lists, gets even cached output.

Peter