Set Host header

Try this:

when HTTP_REQUEST {
    if { [HTTP::header exists X-Forwarded-Host] } {
        HTTP::header replace X-Forwarded-Host [string trim [lindex [split [HTTP::header X-Forwarded-Host] ","] 0]]
    }
}

If the header exists, there's a pretty good bet that it'll have at last one value. If the header has two values delimited with a comma, the above will take the first one. If the header only contains one value, then the split function will produce a list of one, which will still be at the 0 index.