Try to create datadirectory to test whether the .htaccess works

Fixes https://github.com/owncloud/core/pull/11299#issuecomment-56968588 and also https://github.com/owncloud/core/issues/10628 together with https://github.com/owncloud/core/pull/11299#issuecomment-56968588 when backported.

core/setup/controller.php

@@ -116,9 +116,14 @@ class Controller {
 
 		$errors = array();
 
-		if (is_dir($datadir) and is_writable($datadir)) {
+		// Create data directory to test whether the .htaccess works
+		// Notice that this is not necessarily the same data directory as the one
+		// that will effectively be used.
+		@mkdir($datadir);
+		if (is_dir($datadir) && is_writable($datadir)) {
 			// Protect data directory here, so we can test if the protection is working
 			\OC_Setup::protectDataDirectory();
+
 			try {
 				$htaccessWorking = \OC_Util::isHtaccessWorking();
 			} catch (\OC\HintException $e) {