Fixes unreported MoMMI exploit

Created by: 9600bauds

What do you know, it turns out that MoMMIs were able to place plastic bags on their heads, and due to the quality inventory code we have currently, the plastic bag didn't realize it was equipped so it would still allow you to use it as a container. In this way, a MoMMI could theoretically have a backpack and fill it with shit like polyacid smoke grenades or beakers full of reagents for polyacid smoke or replacement polyacid smoke stasis beakers for a polyacid-smoke-creating venom sword! I say "theoretically" because I'm sure that since this bug was unreported for a very long time, it must mean nobody discovered it before so nobody abused it or anything.

Also fixes MoMMIs being unable to take off their plastic bag hat after equipping it, showing it was clearly a bug.