Log a single dash (`-') if referer or user-agent header is missing/unknown

scheme/httpd/logging.scm

@@ -136,7 +136,7 @@
 (define (get-header headers tag)
   (cond
    ((assq tag headers) => cdr)
-   (else "unknown")))
+   (else #f)))
 
 ;; does the log-file rotation on signal USR1
 (define (make-log-file-rotator log-file http-log-lock)
@@ -176,10 +176,10 @@
 		; Files conform to CLF are expected to print the original line.
 		(or http-code "-")
 		(or filesize "-")
-		(if (string? referer) (string-trim referer) "")
+		(if (string? referer) (string-trim referer) '-)
 		(if (string? user-agent) 
 		    (string-trim user-agent char-set:whitespace) 
-		    "")))
+		    '-)))
 
 
 (define (maybe-dns-lookup remote-ip)