feature/session (#3073): Bug fixes

- CI_Session_database_driver::read() didn't pass the session ID to _get_lock()
- CI_Session::unset_userdata() used a wrong key for unsetting when an array is passed to it
diff --git a/system/libraries/Session/Session.php b/system/libraries/Session/Session.php
index a5c9737..be9f5e3 100644
--- a/system/libraries/Session/Session.php
+++ b/system/libraries/Session/Session.php
@@ -567,7 +567,7 @@
 		{
 			foreach ($key as $k)
 			{
-				unset($_SESSION[$key]);
+				unset($_SESSION[$k]);
 			}
 
 			return;