Fix unwanted char promotion in decimal writer with wchar_t

[Mishura4]

In

  it = write_significand(it, dec.significand, significand_size, 1,
                         has_decimal_point ? '.' : Char());

'.' being a prvalue of type char, Char being wchar_t, the result of the ternary is int, leading to write_significand taking on Char=int, leading to a conversion warning down the line with strict warning settings (i.e. /W4 on MSVC).

MSVC warning (relevant line is 67)

[vitaut]

Thanks for the PR. In general we don't care about /W4 warnings since those are mostly noise but this change is simple enough, so I'm OK merging it.

[Mishura4]

You might be thinking of /Wall 🤔which is MSVC's -Weverything

/W4 on msvc is mostly equivalent to -Wall -Wextra -Wconversion on clang