small-optimization2 for mrb_time_zone

mrb_str_new_static is more efficient than mrb_str_new in this case.

mrbgems/mruby-time/src/time.c

@@ -406,7 +406,7 @@ mrb_time_zone(mrb_state *mrb, mrb_value self)
   tm = DATA_GET_PTR(mrb, self, &mrb_time_type, struct mrb_time);
   if (tm->timezone <= MRB_TIMEZONE_NONE) return mrb_nil_value();
   if (tm->timezone >= MRB_TIMEZONE_LAST) return mrb_nil_value();
-  return mrb_str_new(mrb, 
+  return mrb_str_new_static(mrb, 
                             timezone_names[tm->timezone].name,
                             timezone_names[tm->timezone].len);
 }