Update changelog/docs - Some response headers no longer have "http_" prefix.

[guyboertje]

After the switch to Netty in 3.1.X+ some response headers are not prefixed with "http_".

Previously, it appears that Puma (and Mongrel) added this prefix.

Users that depend on a field http_x_forwarded_for to do geoip lookup will have failures as the field is now x_forwarded_for.