Panels ESI: Display arguments concatenation

Function esi_panels_url does an implode of the $display->args to add them to the url. However, under some circumstances these arguments are loaded objects, not strings, leading to a fatal error on the implode. I need to investigate exactly what those are : I suspect panels_everywhere, but might as well be due to a change in panels itself. Meanwhile, checking that they're strings fix the behaviour for now.