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/ball.c

@@ -55,7 +55,7 @@ ballfall(void)
 
         pline_The("iron ball falls on your %s.", body_part(HEAD));
         if (uarmh) {
-            if (is_metallic(uarmh)) {
+            if (hard_helmet(uarmh)) {
                 pline("Fortunately, you are wearing a hard helmet.");
                 dmg = 3;
             } else if (Verbose(0, ballfall))