JSON fix: support roof extruder could be chosen even when roofs weren't enabled (CURA-1723)
JSON fix: support roof extruder could be chosen even when roofs weren't enabled (CURA-1723)

just putting this commit under that issue so that it will be reviewed...

hmflash/Cura,totalretribution/Cura,Curahelper/Cura,Curahelper/Cura,fieldOfView/Cura,ynotstartups/Wanhao,ynotstartups/Wanhao,senttech/Cura,hmflash/Cura,fieldOfView/Cura,senttech/Cura,totalretribution/Cura