message: Fix infinite recursion in test output
diff: @@ -37,7 +37,18 @@ template <typename T>
  struct StringMaker<stdx::optional<T>> {
  static std::string convert(const bsoncxx::stdx::optional<T>& value) {
  if (value) {
  - return StringMaker::convert(value);
  + StringMaker<T>::convert(value.value());
  + }
  +
  + return "{nullopt}";
  + }
  +};
  +
  +template <>
  +struct StringMaker<stdx::optional<bsoncxx::document::view>> {
  + static std::string convert(const bsoncxx::stdx::optional<bsoncxx::document::view>& value) {
  + if (value) {
  + return StringMaker::convert(value.value());
  }
  return "{nullopt}";
  