PHP PanelLayout: Fix getConfig() for 'expanded' config option
PHP PanelLayout: Fix getConfig() for 'expanded' config option

It defaults to true.

Change-Id: I1038d2397c2cccc37efb756151e0865dfca045cf
wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui