Fixing UntypedNode sub json

[wisamidris77]

UntypedNode like UntypedString the result like this "value":{"value":"jo"} instead of "value":"jo"
Before the changes

00:00 +37 -1: test\json_serialization_writer_test.dart: writeUntypedNode [E]
  Expected: '{"fieldName":"name","operator":"contains","value":"jo"}'
    Actual: '{"fieldName":"name","operator":"contains","value":{"value":"jo"}}'
     Which: is different.
            Expected: ... ","value":"jo"}
              Actual: ... ","value":{"value":" ...
                                    ^
             Differ at offset 50

  package:matcher                                 expect
  test\json_serialization_writer_test.dart 186:5  main.<fn>

After

Built test:test.
00:01 +38: All tests passed!

[wisamidris77]

Just copied the wrong output

[baywet]

Thanks for the contribution!

[baywet]

Thank you for making the changes!

[baywet]

@wisamidris77 for this one the CI is failing, would you mind having a look when you have a couple of minutes please?

[ricardoboss]

I think this can be closed. #97 should fix this already. Can you confirm @wisamidris77?