github issue #1060 - crystal helmet

Issue reported by vultur-cadens:  changing helm of brilliance to
crystal made it stop being classified as "hard helmet" so it gave
less protection against things falling onto the hero's head.

Change the is_metallic() tests used on helmets to new hard_helmet().
Unlike when thrown, crystal helmets don't break when objects fall
on them.

Fixes #1060

src/potion.c

@@ -1173,7 +1173,7 @@ peffect_levitation(struct obj *otmp)
                resulted in incrementing 'nothing' */
             gp.potion_nothing = 0; /* not nothing after all */
         } else if (has_ceiling(&u.uz)) {
-            int dmg = rnd(!uarmh ? 10 : !is_metallic(uarmh) ? 6 : 3);
+            int dmg = rnd(!uarmh ? 10 : !hard_helmet(uarmh) ? 6 : 3);
 
             You("hit your %s on the %s.", body_part(HEAD),
                 ceiling(u.ux, u.uy));