Refactor mrb_hash_has_keyWithKey() func in hash.c

If a return value is decided, a program should return value Immediately.
<pre>
if (h) {
  k = kh_get(ht, h, key);
  return mrb_bool_value(k != kh_end(h));
}
</pre>

'result = 0' is ambiguous.
A program returns mrb_false_value().