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/muse.c

@@ -1430,7 +1430,7 @@ find_offensive(struct monst *mtmp)
          */
         nomore(MUSE_SCR_EARTH);
         if (obj->otyp == SCR_EARTH
-            && ((helmet && is_metallic(helmet)) || mtmp->mconf
+            && (hard_helmet(helmet) || mtmp->mconf
                 || amorphous(mtmp->data) || passes_walls(mtmp->data)
                 || noncorporeal(mtmp->data) || unsolid(mtmp->data)
                 || !rn2(10))
@@ -1875,7 +1875,7 @@ rnd_offensive_item(struct monst *mtmp)
     case 0: {
         struct obj *helmet = which_armor(mtmp, W_ARMH);
 
-        if ((helmet && is_metallic(helmet)) || amorphous(pm)
+        if (hard_helmet(helmet) || amorphous(pm)
             || passes_walls(pm) || noncorporeal(pm) || unsolid(pm))
             return SCR_EARTH;
     } /* fall through */