Fix a few more issues

1) Don't use $_SERVER, use Input::instance()->server().  This fixes the problem
   that when you use a browser that doesn't pass in an Accept-Encoding, we'd
   barf on a missing array key

2) Don't bother looking up the _gz key if we don't have gzencode, because we
   probably didn't store one.

3) Only emit the gzip Content-Encoding header if we're actually sending back
   gzipped data.

modules/gallery/controllers/combined.php

@@ -40,8 +40,10 @@ public function css($key) {
    * @param string   the key (typically an md5 sum)
    */
   private function _emit($type, $key) {
+    $input = Input::instance();
+
     // Our data is immutable, so if they already have a copy then it needs no updating.
-    if (!empty($_SERVER["HTTP_IF_MODIFIED_SINCE"])) {
+    if ($input->server("HTTP_IF_MODIFIED_SINCE")) {
       header('HTTP/1.0 304 Not Modified');
       return;
     }
@@ -54,23 +56,20 @@ private function _emit($type, $key) {
     Session::abort_save();
 
     $cache = Cache::instance();
-    if (strpos($_SERVER["HTTP_ACCEPT_ENCODING"], "gzip") !== false ) {
-      $content = $cache->get("{$key}_gz");
-    }
-
-    if (!$content) {
+    $use_gzip = function_exists("gzencode") &&
+      (strpos($input->server("HTTP_ACCEPT_ENCODING"), "gzip") !== false);
+    if ($use_gzip && $content = $cache->get("{$key}_gz")) {
+      header("Content-Encoding: gzip");
+      header("Cache-Control: public");
+    } else {
+      // Fall back to non-gzipped if we have to
       $content = $cache->get($key);
     }
 
-    if (!$content) {
+    if (empty($content)) {
       Kohana::show_404();
     }
 
-    if (strpos($_SERVER["HTTP_ACCEPT_ENCODING"], "gzip") !== false) {
-      header("Content-Encoding: gzip");
-      header("Cache-Control: public");
-    }
-
     // $type is either 'javascript' or 'css'
     header("Content-Type: text/$type; charset=UTF-8");
     header("Expires: Tue, 19 Jan 2038 00:00:00 GMT");