Fix a regression of previously untested behaviour (adapting an item that has
not yet been stored in a store, but has an in-memory powerup installed on it.)

Originally committed by Tristan Seligmann in bzr revision 2748
(<http://bazaar.launchpad.net/~divmod-dev/divmod.org/trunk/revision/2748>).  Accidentally
missed in the conversion to git and this is the easiest way to recover the change.