Add the ABSORBENT flag only if the item does not already have it.

Fixes part of #7090 (the towels not stacking):

When a wet towel dries, it gets the ABSORBENT tag, which is item specific and items with different tags don't stack. The newly made (crafted/found) towel does not have that tag in the item itself (it has the tag "inherited" from the item type, but this is not considered when stacking).

I also changed it to take the item type it reverts to from json.