Alternate Header for  "x-forwarded-proto"

An _x-forwarded-proto_ header which is typically sent by a load balancer that is implementing ssl termination to indicate the request on the public side of the load balancer is secure.

But there are cases (e.g. the combo "IHS" (= IBM Apache Clone) and "WebSphere" ) where an other header is sent instead (e.g.  _$wssc_)

This patch allows to define the name of the header via the property

```
er.extensions.appserver.ERXRequest.xForwardedProtoHeaderKeyForSsl=x-forwarded-proto
```