insert http header in tcp payload

when HTTP_REQUEST {
    if it's a HEAD request, set a variable to reference in SERVER_DATA event
   if { [HTTP::method]=="HEAD" }{
      set request_is_head 1
      log local0. "head"
   }
}
when SERVER_CONNECTED {
    if request was a head, collect the TCP data
   if { [info exists request_is_head] and $request_is_head==1 }{
      log local0. "collecting"
      TCP::collect 15
   }
}
when SERVER_DATA {
    if request was a head, insert a content-length header with a value of 0
   if { [info exists request_is_head] and $request_is_head==1 }{
      log local0. "payload before: [TCP::payload]"
      log local0. "index: [string first "\r\n\r\n" $server_payload]"
      TCP::payload replace [string first "\r\n\r\n" [TCP::payload]] 0 "\r\nContent-length: 0" 
      log local0. "payload after: [TCP::payload]"
   }
}