TimeSpan serialization bug

Nasty silent bug this one. The default resolver incorrectly handles the `TimeSpan` type.

To reproduce:  

```
var timeSpan1 = TimeSpan.Parse("00:03:14.008615");
byte[] bytes = JsonSerializer.Serialize(timeSpan1);
var timeSpan2 = JsonSerializer.Deserialize<TimeSpan>(bytes);
```

timeSpan1 = "00:03:14.008615"
timeSpan2 = "00:03:14.8615"