set correct content type for JSON requests

lam/templates/misc/ajax.php

@@ -93,7 +93,7 @@ class lamAjax {
 	/**
 	 * Sets JSON HTTP header.
 	 */
-	private function setHeader() {
+	private static function setHeader() {
 		if (!headers_sent()) {
 			header('Content-Type: application/json; charset=utf-8');
 		}