Previously Etherpad would not pass the correct client IP address through and this caused the rate limiter to limit users behind reverse proxies. This change allows Etherpad to use a client IP passed from a reverse proxy.
Note to devs: This header can be spoofed and spoofing the header could be used in an attack. To mitigate additional *steps should be taken by Etherpad site admins IE doing rate limiting at proxy.* This only really applies to large scale deployments but it's worth noting.
