Adds Alternative X_FORWARDED_PROTO header used by haproxy loadbalancer (Rightscale)

I am deploying a Meteor App to production at a Fortune 100 co with a rigid DevOps policy (no changes to existing infrastructure). The HAPROXY managed by RightScale uses the Capitalised_underscore_separated header X_FORWARDED_PROTO for re-routing traffic to application servers. This simple change to force-ssl will allow other apps to deploy using HAPROXY without any modification. (Thanks!)