Fix Session connect to DB

[Igor]

In this situation create new connect to DB. Don't use exists of default group.

Code:

	public function open($path, $name)
	{
		if (Config::item('database.'.$this->db_group) === NULL)
		{
			// There's no defined group, use the default database
			$this->db = new Database;
		}
....

Need change to:

			// There's no defined group, use the default database
                        $this->db = Database::instance();

[Geert]

Fixed by Oscar in r2692