Fix cdata handling in `XML::writeElement()`
Fix cdata handling in `XML::writeElement()`

See #2545

WoltLab/WCF,Cyperghost/WCF,Cyperghost/WCF,WoltLab/WCF,MenesesEvandro/WCF,SoftCreatR/WCF,Cyperghost/WCF,SoftCreatR/WCF,WoltLab/WCF,MenesesEvandro/WCF,WoltLab/WCF,Cyperghost/WCF,SoftCreatR/WCF,SoftCreatR/WCF,Cyperghost/WCF