"VI" is the ISO_3166-2 code for US Virgin Islands but this gem uses it for Vietnam

[martingjaldbaek]

"VI" is the ISO_3166-2 code for US Virgin Islands source
"VN" is the ISO_3166-2 code for Vietnam source

In this gem the region code "VI" is used for Vietnam, and the code "VN" is not used. Looking through the documentation, I couldn't find a decision to follow the ISO 3166-1 alpha-2 standard, but I couldn't find any decision not to either - I haven't actually found any definition of the country codes as such. But looking at them in practice they seem to mostly follow the ISO 3166-1 alpha-2 standard anyway, which can easily lead someone (i.e. me) to wrongly assume that it is the intention, and run into trouble when cross-referencing with other sources of information. As such I would argue that almost following the standard is worse than breaking with it completely.

I would suggest that a decision be made to either follow the standard or not - and document the decision in either case. And then either change vi.yaml to vn.yaml, or add a warning that this is purposely not following the ISO standard, maybe document the cases in which it has chosen to diverge.