Make wood and cactus armor usable as fuel

[Wuzzy2]

Wood armor is made of wood and cactus armor is made of cactus. Both wood and armor are usable as furnace fuel, but not wood armor and cactus armor.

Please make those items usable as furnace fuel, thanks (with an appropriate/logical burntime, of course, see https://wiki.minetest.net/Smelting for comparison). :-)

[orbea]

@Wuzzy2 https://github.com/minetest-mods/3d_armor/pull/7