Use X-Accel-Buffering to disable NGINX response buffering on all non-Rails requests
From @nick.thomas 's suggestion in https://gitlab.com/gitlab-org/gitlab-ce/issues/24497#note_18973514
Longer explanation: we only want NGINX response buffering for requests handled by Unicorn: it frees up Unicorn workers faster. For large requests not handled by Unicorn, response buffering is not necessary and it can actually be problematic, for instance when a large repository is cloned more than once at the same time. This can create a situation where the NGINX buffers fill up too fast.