Fixed underflow on DateTime parsing

[sunghwan2789]

Given system that time zone is configured to -01:00, HotChocolate and StrawberryShake are parsing "0001-01-01T00:00:00.000Z" to "0001-01-02T00:00:00.000+00:00".

This pr fixes default(DateTime) is parsed to non-default value

[glen-84]

I don't know why the special handling of Z exists. @michaelstaib, do you remember?

If not, I think that we can also remove it from the DateTimeSerializer class.

[michaelstaib]

@glen-84 simply check if tests break when you remove it :)

[glen-84]

@sunghwan2789

Would you like to apply a similar change to DateTimeSerializer#TryDeserializeFromString? (i.e. remove the special handling of Z, and just use DateTimeOffset.TryParse)