Move sending headers before the flush, fixes #1746

There was
Warning: Cannot modify header information - headers already sent in action.php on line 228
see https://piwigo.org/forum/viewtopic.php?id=32254

Unclear how headers could had been sent before sending headers, but..

action.php

@@ -216,6 +216,11 @@ if (ini_get('safe_mode') == 0)
   @set_time_limit(0);
 }
 
+foreach ($http_headers as $header)
+{
+  header( $header );
+}
+
 // Without clean and flush there may be some image download problems, or image can be corrupted after download
 if (ob_get_length() !== FALSE)
 {
@@ -223,11 +228,6 @@ if (ob_get_length() !== FALSE)
 }
 flush();
 
-foreach ($http_headers as $header)
-{
-  header( $header );
-}
-
 @readfile($file);
 
 ?>