If you try to get a HTTP header via Request::header('HTTP_USER_AGENT') when the request was made through a test case, it always returns "NULL". If the request was made through the browser everything works fine.
I checked in the symfony source code wether the headers were set (when running the test).
var_dump($server):
array(4) {
'HTTP_HOST' =>
string(9) "localhost"
'HTTP_USER_AGENT' =>
string(19) "Symfony2 BrowserKit"
'HTTPS' =>
bool(false)
}