Fix flaky Unmarshal test

The JSON unmarshaling test is flaky:

=== RUN   TestUnmarshal/{"foo":"bar","arr":[1,"2"]}
    marshal_test.go:58:
                Error Trace:    /Users/timo/work/gitlab/moa/value/marshal_test.go:58
                Error:          Not equal:
                                expected: "{\"foo\":\"bar\",\"arr\":[1,\"2\"]}"
                                actual  : "{\"arr\":[1,\"2\"],\"foo\":\"bar\"}"

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -{"foo":"bar","arr":[1,"2"]}
                                +{"arr":[1,"2"],"foo":"bar"}
                Test:           TestUnmarshal/{"foo":"bar","arr":[1,"2"]}