Allow public auth to recognize sesssion

When a public link password has been input, its auth is stored in the
session.
This fix makes it possible to recognize the session when using public
webdav from the files UI.

apps/files_sharing/lib/connector/publicauth.php

@@ -82,6 +82,9 @@ class PublicAuth extends \Sabre\DAV\Auth\Backend\AbstractBasic {
 
 					}
 					return true;
+				} else if (\OC::$server->getSession()->exists('public_link_authenticated')
+					&& \OC::$server->getSession()->get('public_link_authenticated') === $linkItem['id']) {
+					return true;
 				} else {
 					return false;
 				}