Fixed DateTime serialization with JSON using single quotes

[EdwardCooke]

Fixes #587

When using JsonCompatible we will now register a DateTimeConverter.

If the user is adding a DateTimeConverter object before calling JsonCompatible on the serializer, this call will fail. If the user is adding some other, custom, DateTime converter, the one added by JsonCompatible will take priority, and the users converter will be skipped.

If the user wants to specify their own DateTimeConverter or some other DateTime converter object, they will need to do it after calling JsonCompatible.

If they are passing their own DateTimeConverter, they can now specify the optional constructor argument, doubleQuotes, so it will dish out valid JSON.

[stan-sz]

@EdwardCooke - my local test runs fail with

[xUnit.net 00:00:02.63]     JsonCompatible_EncaseDateTimesInDoubleQuotes [FAIL]
  Failed JsonCompatible_EncaseDateTimesInDoubleQuotes [7 ms]
  Error Message:
   Expected string to contain equivalent of "{\"DateTime\": \"01/14/2023 00:01:02\"}" but found "{\"DateTime\": \"01/13/2023 23:01:02\"}"

My guess is that the constructor in the test case is missing the DateTimeKind as:

var testObject = new TestObject { DateTime = new DateTime(2023, 01, 14, 0, 1, 2, DateTimeKind.Utc) };

[EdwardCooke]

This flaky test will be fixed in the next release.