Make Value( bool value ) etc explicit

when passing a Json::Value pointer to a const Json::Value &;
compiler will automatically transform the pointer to bool then to a json::value;

void LogJson(const Json::Value& value) {
  LOG(ERROR) << value.toStyledString();
}
int main() {
  Json::Value json = Json::Value(Json::objectValue)
  const Json::Value *json_p = &json;
  LogJson(json);
  LogJson(json_p);
}

the first LogJson will log a "{}"
the second LogJson will log a "true", caused by implict conversation

Expecte the second LogJson will compile fail.

Start at the Json::Value constructors and conversion behavior demonstrated by the LogJson example. Reproduce both calls with the shown snippet, then verify that passing a Json::Value pointer no longer compiles while passing a Json::Value object still does.