Map not int enum to correct underlying_type

[agmt]

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

This PR correctly prints values of such enums:
enum E{ I = 5000000000ULL };

Test update is needed because "0" may not detect an error.

[vitaut]

Thanks for the PR!

[vitaut]

Is it actually formatted differently if we don't map to underlying type?

[agmt]

I don't understand the problem clearly but if map() returns short (which doesn't enumerated in enum type)

FMT_ENABLE_IF(std::is_enum<T>::value &&
                          !has_formatter<T, Context>::value &&
                          !has_fallback_formatter<T, Context>::value)>
  FMT_CONSTEXPR typename std::underlying_type<T>::type map(const T& val) {
    return static_cast<typename std::underlying_type<T>::type>(val);
  }

then after that will be called map(short) with correct argument and map(int) with garbage ( =0 almost always). It means that fmt::format(short_enum_value) will return "0" regardless of the argument.

[vitaut]

Anyway, I merged the PR and will add a regression test separately. Thanks!