Fix test using private propertries

apps/dav/lib/Server.php

@@ -47,7 +47,7 @@ class Server {
 	private $baseUri;
 
 	/** @var Connector\Sabre\Server  */
-	protected $server;
+	private $server;
 
 	public function __construct(IRequest $request, $baseUri) {
 		$this->request = $request;

apps/dav/tests/unit/ServerTest.php

@@ -39,6 +39,6 @@ public function test() {
 		$r = $this->getMockBuilder('\OCP\IRequest')
 			->disableOriginalConstructor()->getMock();
 		$s = new Server($r, '/');
-		$this->assertNotNull($s->server);
+		$this->assertInstanceOf('OCA\DAV\Server', $s);
 	}
 }