Add retry-after header into HAProxy

From the discussion in #795 (closed), we managed to make most of headers in HAProxy and RackAttack matched. There is only one left: retry-after. This header is a HTTP standard (mentioned in MDN and RFC). Rack attack already supports that. So, HAProxy should properly support that too. The priority of this issue is quite low, mostly because:

• The clients are looking at different headers (python, npm, and go-gitlab). Thanks @cmiskell.
• The likeliness of hitting rate limiting at HAProxy is much lower than RackAttack.
• This issue will be closed if we move forward to migrate rate limiting to Cloudflare