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/hack.c

@@ -2945,7 +2945,7 @@ spoteffects(boolean pick)
                   ceiling(u.ux, u.uy));
             if (mtmp->mtame) { /* jumps to greet you, not attack */
                 ;
-            } else if (uarmh && is_metallic(uarmh)) {
+            } else if (hard_helmet(uarmh)) {
                 pline("Its blow glances off your %s.",
                       helm_simple_name(uarmh));
             } else if (u.uac + 3 <= rnd(20)) {