get effective username after webdav login, fixes oc-2108

lib/connector/sabre/auth.php

@@ -37,7 +37,7 @@ class OC_Connector_Sabre_Auth extends Sabre_DAV_Auth_Backend_AbstractBasic {
 		} else {
 			OC_Util::setUpFS();//login hooks may need early access to the filesystem
 			if(OC_User::login($username, $password)) {
-				OC_Util::setUpFS($username);
+				OC_Util::setUpFS(OC_User::getUser());
 				return true;
 			}
 			else{