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/dig.c

@@ -1459,7 +1459,7 @@ zap_dig(void)
                 }
                 You("loosen a rock from the %s.", ceiling(u.ux, u.uy));
                 pline("It falls on your %s!", body_part(HEAD));
-                dmg = rnd((uarmh && is_metallic(uarmh)) ? 2 : 6);
+                dmg = rnd(hard_helmet(uarmh) ? 2 : 6);
                 losehp(Maybe_Half_Phys(dmg), "falling rock", KILLED_BY_AN);
                 otmp = mksobj_at(ROCK, u.ux, u.uy, FALSE, FALSE);
                 if (otmp) {