arguments() contains empty keys when existing short-only options

[ldeng-ustc]

When use a short-only options name, ParseResult::arguments.key() will be empty string, because only long_name will be insert into m_sequential in parse_option:

// line 2164~2178
inline
void
OptionParser::parse_option
(
  const std::shared_ptr<OptionDetails>& value,
  const std::string& /*name*/,
  const std::string& arg
)
{
  auto hash = value->hash();
  auto& result = m_parsed[hash];
  result.parse(value, arg);

  m_sequential.emplace_back(value->long_name(), arg); // empty key could be insert into m_sequential when option is short-only name.
}

Also, arguments_string() will output parse results with no name like (where I set short-only option n=16 and m=8):

 = 16
 = 8
help = false (default)
long = false (default)

Possible solution: Add essential_name() to OptionsDetails, which return long name, or short name when long name is empty. And use these as key in m_sequential and m_defaults

Maybe I will create a PR to solve these.