make from_chars(double) parsing locale-independent

[metsw24-max]

Locale-dependent quality value parsing

detail::from_chars(double) hands off to std::strtod, which reads the decimal point from the global C locale, whereas the integer overload beside it is written to stay locale-independent. Once an embedding process has run setlocale(LC_ALL, "") into a locale that uses ',' (de, fr and others), an untrusted Accept/Accept-Encoding header such as gzip;q=0.8 is parsed as q=0 because strtod stops at the '.', so the weight collapses to "not acceptable" and the server quietly drops gzip and skews content negotiation. The fix translates '.' to the active locale's separator before the call, restoring the locale-independent intent without pulling in a different parser. The regression test pins q-value ordering under a comma-decimal locale and skips where none is installed.

[yhirose]

@metsw24-max thanks for the pull request! I implemented it in a better way.