Fix mod.constants cannot return the single character constant

src/variable.c

@@ -963,7 +963,7 @@ const_i(mrb_state *mrb, mrb_sym sym, mrb_value v, void *p)
 
   ary = *(mrb_value*)p;
   s = mrb_sym2name_len(mrb, sym, &len);
-  if (len > 1 && ISUPPER(s[0])) {
+  if (len >= 1 && ISUPPER(s[0])) {
     mrb_ary_push(mrb, ary, mrb_symbol_value(sym));
   }
   return 0;