Allow ContentElement/Module using ArrayObject for $arrData

I use the ArrayObject class for storing the $arrData in a content element, because I want to integrate some values from a different source which shall automatically passed to the Template object. 

It works almost, only the css class got lost, because cssID[1] is overriden and Template::$arrData and ContentElement::$arrData are references now. By changing 2 lines, this issue would be fixed.