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/trap.c

@@ -1197,7 +1197,7 @@ trapeffect_rocktrap(
                     pline("Unfortunately, you are wearing %s.",
                           an(helm_simple_name(uarmh))); /* helm or hat */
                     dmg = 2;
-                } else if (is_metallic(uarmh)) {
+                } else if (hard_helmet(uarmh)) {
                     pline("Fortunately, you are wearing a hard helmet.");
                     dmg = 2;
                 } else if (Verbose(3, trapeffect_rocktrap)) {