test: add tests for datetimeoffset converters

[JamieMagee]

FYI: @tobysmith568

[tobysmith568]

Ah looks good @JamieMagee. Is it worth adding a test to each converter with a datetime in this format 2022-01-13T09:21:34.000+00:00 as it's also a format that the GitHub API sometimes returns? I also wonder if the unix timestamps in their tests should be ints rather than strings?

[JamieMagee]

I added tests for the other format. Unfortunately, JsonSerializer.Deserialize only takes string as a parameter so I can't pass an int directly, but I am passing the JSON equivalent of an int @"123" instead of @"""123"""

[tobysmith568]

Ah yeah that makes sense :)

[JamieMagee]

Super! I've merged this and cut a release with your changes 🎉