Merge pull request #4788 from shuujii/fix-argument-specs-to-Exception

Fix argument specs to `Exception`

src/error.c

@@ -594,7 +594,7 @@ mrb_init_exception(mrb_state *mrb)
   mrb->eException_class = exception = mrb_define_class(mrb, "Exception", mrb->object_class); /* 15.2.22 */
   MRB_SET_INSTANCE_TT(exception, MRB_TT_EXCEPTION);
   mrb_define_class_method(mrb, exception, "exception", mrb_instance_new,  MRB_ARGS_OPT(1));
-  mrb_define_method(mrb, exception, "exception",       exc_exception,     MRB_ARGS_ANY());
+  mrb_define_method(mrb, exception, "exception",       exc_exception,     MRB_ARGS_OPT(1));
   mrb_define_method(mrb, exception, "initialize",      exc_initialize,    MRB_ARGS_OPT(1));
   mrb_define_method(mrb, exception, "to_s",            exc_to_s,          MRB_ARGS_NONE());
   mrb_define_method(mrb, exception, "message",         exc_message,       MRB_ARGS_NONE());