Fix caching of menu in manager

Until now probably manager menu never been cached by missing context key in path to cache file.

Take a look on **'mgr/menus/'** in https://github.com/modxcms/revolution/blob/release-2.2/manager/controllers/default/header.php#L12
```php
$menus = $modx->cacheManager->get('mgr/menus/'.$modx->getOption('manager_language',null,$modx->getOption('cultureKey',null,'en')), array(
    xPDO::OPT_CACHE_KEY => $modx->getOption('cache_menu_key', null, 'menu'),
    xPDO::OPT_CACHE_HANDLER => $modx->getOption('cache_menu_handler', null, $modx->getOption(xPDO::OPT_CACHE_HANDLER)),
    xPDO::OPT_CACHE_FORMAT => (integer) $modx->getOption('cache_menu_format', null, $modx->getOption(xPDO::OPT_CACHE_FORMAT, null, xPDOCacheManager::CACHE_PHP)),
));
```