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/mhitu.c

@@ -526,7 +526,7 @@ mattacku(register struct monst *mtmp)
                 return 0; /* lurkers don't attack */
 
             obj = which_armor(mtmp, WORN_HELMET);
-            if (obj && is_metallic(obj)) {
+            if (hard_helmet(obj)) {
                 Your("blow glances off %s %s.", s_suffix(mon_nam(mtmp)),
                      helm_simple_name(obj));
             } else {