Do not try to set a material's metadata when we know it has not changed
Do not try to set a material's metadata when we know it has not changed

Contributes to CURA-342

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