Allow using "/" in "overwritewebroot"

Whenever the reverse proxy is using "/" as the webroot, it is now
possible to set that value in "overwritewebroot"

Fixes #7870 

Please review @internalfx @fanchthesystem
You need to set "/" for "overwritewebroot" to make it work.

I've tested this with both "/" and another value like "/ocproxy" in a reverse proxy environment.