We're still relying on global headers instead of using the request/response objects in places like page cache handling. This needs to be fixed before release, so here's a start on it.
Working on cleaning up global variable calls such as $_SERVER in code #1998638: [Meta] Refactor raw PHP variables (e.g $_SERVER, $_REQUEST, $_GET, $_POST) with Symfony Request object.
Not the appropriate place to discuss #1855260-39: Make sure page caching works with accept header-based routing.
Update drupal_add_http_header() so relying on Symfony to send headers patch: reqrep-1984766-9.patch result: "FAILED: [[SimpleTest]]: [MySQL] 55,672 pass(es), 29 fail(s), and 0 exception(s)." details.
- drupal_set_page_cache() and drupal_serve_page_from_cache() need clean-up.
- drupal_add_http_header() still being used in install.core.inc and cannot be removed yet.
- Update or add new patch to use symfony to send headers.