message: serialization: Robustify test.
diff: @@ -1442,6 +1442,7 @@ json_optionals()
  serializer.serialize("value", o);
  }
  auto json = elle::json::read(stream);
  + BOOST_REQUIRE(json.type() == typeid(elle::json::Object));
  BOOST_CHECK_EQUAL(boost::any_cast<elle::json::Object&>(json).size(), 0);
  }
  }
  