Fix cache control

Multiple values should be seperated by comma

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control

Closes #156

Author: nitriques

lib/image.php

@@ -249,7 +249,7 @@ function __errorHandler($errno=NULL, $errstr, $errfile=NULL, $errline=NULL, $err
 		$etag = md5($last_modified . $image_path);
 		// Use configured max-age or fallback on 3 days (See #88)
 		$maxage = isset($settings['image']['max-age']) ? $settings['image']['max-age'] : 86400;
-		$cacheControl = 'public; max-age=' . $maxage;
+		$cacheControl = 'public, max-age=' . $maxage;
 
 		// Add no-transform in order to prevent ISPs to
 		// serve image over http through a compressing proxy