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/drivers/Session_database_driver.php b/system/libraries/Session/drivers/Session_database_driver.php
index 032199f..563d1fd 100644
--- a/system/libraries/Session/drivers/Session_database_driver.php
+++ b/system/libraries/Session/drivers/Session_database_driver.php
@@ -116,7 +116,7 @@
 
 	public function read($session_id)
 	{
-		if ($this->_get_lock() !== FALSE)
+		if ($this->_get_lock($session_id) !== FALSE)
 		{
 			$this->_db
 				->select('data')