GELF HTTP X-Forwarded-For headers not honored

## What?

As a Graylog user utilizing the GELF HTTP input I expect the X-Forwarded-For headers to be honored when using a load balancer.

## Why?

Currently when using any load balancer when explicitly stating the headers in Graylog they are not honored, as a result the IP/client address shows as coming from the load balancer and not the actual client.

See HS-565348230 for details.