Fix so imap folder attribute comparisons are case-insensitive (#1490466)

+ make in_array_nocase() much faster for ASCII strings

program/include/rcmail.php

@@ -1586,7 +1586,7 @@ class rcmail extends rcube
             // skip folders in which it isn't possible to create subfolders
             if (!empty($opts['skip_noinferiors'])) {
                 $attrs = $this->storage->folder_attributes($folder['id']);
-                if ($attrs && in_array('\\Noinferiors', $attrs)) {
+                if ($attrs && in_array_nocase('\\Noinferiors', $attrs)) {
                     continue;
                 }
             }