How to preserve data in a HTTP 302 redirect of a POST

when RULE_INIT {
    set static::ext_url "http://host.header.com/path/login.php"
}
when HTTP_REQUEST {
    Check if request was a POST
   if { [string tolower [HTTP::method]] eq "post" } {
       Check if there is a Content-Length header
      if { [HTTP::header exists "Content-Length"] } {
         if { [HTTP::header "Content-Length"] > 1048000 }{
         Content-Length over 1Mb so collect 1Mb
            set content_length 1048000
     } else {
         Content-Length under 1Mb so collect actual length
            set content_length [HTTP::header "Content-Length"]
         }
      } else {
      Response did not have Content-Length header, so use default of 1Mb
         set content_length 1048000
       }
        Don't collect content if Content-Length header value was 0
       if { $content_length > 0 } {
          HTTP::collect $content_length
       }
    }    
}
when HTTP_REQUEST_DATA {
    set content "?"       
    foreach p [split [HTTP::payload] &] {
        set name  [URI::decode [getfield $p = 1]]
        set value [URI::decode [getfield $p = 2]]
    set content "${content}$name=$value&"
    log local0. "Content: $content"
    }
    set content "${content}"
    set where_to "$static::ext_url$content"
    log local0. "$where_to"
}
when HTTP_RESPONSE {
    if { [info exists where_to] } {
        HTTP::respond 302 Location $where_to
    }
}