FIX: Download of files named "true" or "false" impossible

It was not possible to download a file called "true" or "false" (caseinsensitive) when clicking the link "download" due to the result of json_decode().
Both cases are now implicitly checked.

Hotfix provided by Herbert Pophal

apps/files/ajax/download.php

@@ -34,8 +34,9 @@ $files = $_GET["files"];
 $dir = $_GET["dir"];
 
 $files_list = json_decode($files);
+
 // in case we get only a single file
-if ($files_list === NULL ) {
+if ($files_list === NULL || $files_list === true || $files_list === false) {
 	$files_list = array($files);
 }