Small fix Mastodon share

$a['method'] can be undefined.
https://github.com/FreshRSS/FreshRSS/pull/1674
https://github.com/FreshRSS/FreshRSS/issues/1521

app/FreshRSS.php

@@ -112,7 +112,7 @@ class FreshRSS extends Minz_FrontController {
 		switch (Minz_Request::controllerName()) {
 			case 'index':
 				$urlToAuthorize = array_filter(array_map(function ($a) {
-					if ('POST' === $a['method']) {
+					if (isset($a['method']) && $a['method'] === 'POST') {
 						return $a['url'];
 					}
 				}, FreshRSS_Context::$user_conf->sharing));