New serializers are sensitive to locale

[ewoutkramer]

Torben Hagensen reported on Zulip (https://chat.fhir.org/#narrow/stream/179171-dotnet/topic/Serializing.20instant.20elements) that the new serializers are formatting the Instant type incorrectly, and indeed it is easy to reproduce that our serialization logic is sensitive to locale:

CultureInfo.CurrentCulture = new CultureInfo("da");

string FMT_FULL = "yyyy-MM-dd'T'HH:mm:ss.FFFFFFFK";
var now = DateTimeOffset.Now;

// Returns 2023-10-26T13.27.49.0572969+02:00
now.ToString(FMT_FULL).Dump();

// Returns 2023-10-26T13:27:49.0572969+02:00
XmlConvert.ToString(now,FMT_FULL).Dump();

It turns out that in the case of DateTimeOffset.ToString(), we run into localization issues, even though we fully specify the output format. This behaviour is different from the XmlConvert.ToString(), which we have used before.

The solution is to explicitly tag the invariant culture along:

// Returns 2023-10-26T13:27:49.0572969+02:00
now.ToString(FMT_FULL, CultureInfo.InvariantCulture).Dump();

I am pretty sure this logic is present in other parts of the new System types, and newer serializers so we should ensure we look a bit further than just the serializers.