Improve `is` description for invisible differences

If the expected and the actual object differ in things that are not visible in JSON format, _hamjest_ prints something like

```
Expected: is      {"a":1,"e":"2","f":{},"g":{"h":3},"k":[4,5]}
but: was          {"a":1,"e":"2","f":{},"g":{"h":3},"k":[4,5]}
```

which is not very helpful. But it might be possible to detect this and fall back to a non-JSON description of the differences.
