Adds ESI/1.0 capability to the given Request.
Request $request A Request instance:
public function addSurrogateEsiCapability(Request $request) {
$current = $request->headers
->get('Surrogate-Capability');
$new = 'symfony2="ESI/1.0"';
$request->headers
->set('Surrogate-Capability', $current ? $current . ', ' . $new : $new);
}