message: white_check_mark: improved test coverage
  As we guarantee proper before, we do not need to cope with it later.
diff: @@ -6415,12 +6415,8 @@ class serializer
  if (ensure_ascii and (s[i] & 0x80 or s[i] == 0x7F))
  {
  const auto bytes = bytes_following(static_cast<uint8_t>(s[i]));
  - if (bytes == std::string::npos)
  - {
  - // invalid characters are treated as is, so no
  - // additional space will be used
  - break;
  - }
  + // invalid characters will be detected by throw_if_invalid_utf8
  + assert (bytes != std::string::npos);
  if (bytes == 3)
  {
  @@ -6588,12 +6584,8 @@ class serializer
  (ensure_ascii and (s[i] & 0x80 or s[i] == 0x7F)))
  {
  const auto bytes = bytes_following(static_cast<uint8_t>(s[i]));
  - if (bytes == std::string::npos)
  - {
  - // copy invalid character as is
  - result[pos++] = s[i];
  - break;
  - }
  + // invalid characters will be detected by throw_if_invalid_utf8
  + assert (bytes != std::string::npos);
  // check that the additional bytes are present
  assert(i + bytes < s.size());
  