message: Correct the return type on two format overloads
diff: @@ -5128,7 +5128,7 @@ format(const std::basic_string<CharT, Traits>& fmt, const sys_time<Duration>& tp
  }
  template <class CharT, class Traits, class Rep, class Period>
  -std::basic_string<CharT>
  +std::basic_string<CharT, Traits>
  format(const std::locale& loc, const std::basic_string<CharT, Traits>& fmt,
  const std::chrono::duration<Rep, Period>& d)
  {
  @@ -5139,7 +5139,7 @@ format(const std::locale& loc, const std::basic_string<CharT, Traits>& fmt,
  }
  template <class CharT, class Traits, class Rep, class Period>
  -std::basic_string<CharT>
  +std::basic_string<CharT, Traits>
  format(const std::basic_string<CharT, Traits>& fmt,
  const std::chrono::duration<Rep, Period>& d)
  {
  