fix github issue #1246 - fading spellbook

Issue reported by chadministratorwastaken:  persistent inventory
window didn't update if reading a spellbook caused that book to fade
to blank paper.

Issue was reported for 'curses' but it was general.  The core made
the possibly-revised book type become discovered but that wouldn't
update perm_invent unless the type of book hadn't been discovered.
So if plain spellbook wasn't known yet, the persistent inventory
window did update as intended, but if spellbook of blank paper was
already discovered, the window continued to show the old book type
instead of changing to reflect the revised type (until subsequent
activity triggered another perm_invent update).

Fixes #1246

doc/fixes3-7-0.txt

@@ -567,6 +567,8 @@ perm_invent: when buying shop goods using itemized purchasing while persistent
 	soon as any item was bought (actual item-by-item purchase worked ok)
 perm_invent: making an engraving which reduced known enchantment of a weapon
 	or known charge count of a marker didn't update persistent inventory
+perm_invent: over-reading a spellbook so that in faded to blank didn't update
+	persistent inventory to show that if blank spellbook was already known
 change getloc fastmove keys in number_pad mode from hardcoded HJKL to the
 	run/rush movement keys (meta+number)
 allow using rush/run prefix key in getloc to fastmove the cursor