Add missing maintenance plugin to new DAV endpoint

The `/remote.php/dav/` endpoint was not implementing the MaintenancePlugin. Thus when the instance was put into maintenance mode the endpoints were still accessible and delivered empty content. Sync clients really do love this.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

apps/dav/lib/Server.php

@@ -74,6 +74,9 @@ class Server {
 		$root = new RootCollection();
 		$this->server = new \OCA\DAV\Connector\Sabre\Server($root);
 
+		// Add maintenance plugin
+		$this->server->addPlugin(new \OCA\DAV\Connector\Sabre\MaintenancePlugin(\OC::$server->getConfig()));
+
 		// Backends
 		$authBackend = new Auth(
 			\OC::$server->getSession(),