fix(CultureInfo): default locale name should contain hyphen rather than underscore

While implementing #156, I noticed something was amiss with LocaleDateTime. For my locale (en-GB), I should see the date in the format dd/MM/yyyy, but what I actually saw in the REPL was:

>>> from pyoda_time import LocalDateTime
>>> LocalDateTime(year=1, month=2, day=3)
2/3/0001 12:00:00 AM

Looking into it, I discovered that CultureInfo.current_culture.date_time_format.short_date_pattern was giving me M/d/yyyy instead of dd/MM/yyyy.

This was made more confusing by the fact that if I constructed a CultureInfo manually, it gave me the right short date format:

>>> from pyoda_time._compatibility._culture_info import CultureInfo
>>> CultureInfo("en-GB").date_time_format.short_date_pattern
'dd/MM/yyyy'

After a bit more investigation, I discovered that the root cause lay in CultureInfo._get_default_locale_name(), which was returning en_GB (underscore) rather than en-GB (hyphen). Verified via:

>>> CultureInfo("en_GB").date_time_format.short_date_pattern
'M/d/yyyy'

This in itself was perplexing as I would have expected both "en-GB" and "en_GB" to result in Cultures with the same short date format. I went hunting in dotnet to get some answers... This test passes:

    [Test]
    public void EnGbShortDateFormat()
    {
        Assert.That(CultureInfo.CurrentCulture.DateTimeFormat.ShortDatePattern, Is.EqualTo("dd/MM/yyyy"));
        Assert.That(new CultureInfo("en-GB").DateTimeFormat.ShortDatePattern, Is.EqualTo("dd/MM/yyyy"));
        Assert.That(new CultureInfo("en_GB").DateTimeFormat.ShortDatePattern, Is.EqualTo("M/d/yyyy"));
    }

So the take-aways here are:

Yes, there was a bug in CultureInfo._get_default_locale_name() which for me was returning "en_GB" (ICU locale name) rather than "en-GB" (dotnet style culture name).
There isn't a bug on our side with the short date formats for "en_GB" and "en-GB" cultures having different short date formats; Or at least, we are bug-for-bug compatible with dotnet in this regard.

Oh, and after patching this up, here is what I get in my REPL:

>>> from pyoda_time import LocalDateTime
>>> LocalDateTime(year=1, month=2, day=3)
03/02/0001 00:00:00

chrisimcevoy / pyoda-time

fix(CultureInfo): default locale name should contain hyphen rather than underscore #157

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