Fix localized format for float-point numbers

[ShawnZhong]

Fix #3263

The following piece of code used to print 15, 9, 14, which should be 15, 15, 15. This PR fixes this issue.

#include <locale>
#include <fmt/format.h>

template <typename Char> struct numpunct : std::numpunct<Char> {
 protected:
  Char do_decimal_point() const override { return '.'; }
  std::string do_grouping() const override { return "\1"; }
  Char do_thousands_sep() const override { return ','; }
};

int main() {
  auto loc = std::locale(std::locale(), new numpunct<char>());
  auto i = fmt::format(loc, "{:15.6Lf}", 0.1).size();
  auto j = fmt::format(loc, "{:15.6Lf}", 1.0).size();
  auto k = fmt::format(loc, "{:15.6Lf}", 1e3).size();
  fmt::print("{}, {}, {}", i, j, k);
}

[vitaut]

Thank you!