Hidden (object-form) mimics aren't hit by thrown objects

This is both a bugfix (the hero would be unlikely to aim their
throw at what appeared to be an object) and a balance fix (it was
possible to, somewhat tediously, defend yourself from mimics by
throwing gold pieces at them, and for many players this became
standard strategy in shops, negating the threat from mimics).

src/zap.c

@@ -3957,9 +3957,15 @@ bhit(
            give message and skip it in order to keep going;
            if attack is light and mtmp is a mimic pretending to be an
            object, behave as if there is no monster here (if pretending
-           to be furniture, it will be revealed by flash_hits_mon()) */
+           to be furniture, it will be revealed by flash_hits_mon());
+           thrown objects don't hit mimics pretending to be objects (both
+           because the hero is likely aiming to throw over what seems to
+           be an object rather than at it, and for balance because
+           otherwise mimics are too easy to identify by throwing gold at
+           them) */
         if (mtmp && (((weapon == THROWN_WEAPON || weapon == KICKED_WEAPON)
-                      && shade_miss(&gy.youmonst, mtmp, obj, TRUE, TRUE))
+                      && (shade_miss(&gy.youmonst, mtmp, obj, TRUE, TRUE)
+                          || M_AP_TYPE(mtmp) == M_AP_OBJECT))
                      || (weapon == FLASHED_LIGHT
                          && M_AP_TYPE(mtmp) == M_AP_OBJECT)))
             mtmp = (struct monst *) 0;