rack: SSL Env vars can only be in Request.env

This reverts commit c702f76b271515e9c42dcb923d379fbfac4c83cd and turns it
into a documentation only fix. As it turns out, ENV should have never been
used at all, as the Rack docs say nothing about it *and* Passenger's
behaviour in 2.2.3 + 2.2.4 was completely broken and still is half-way broken
in 2.2.5 (but is OK with the Rack specs).