TST: raise an error json serialization of floats that cannot be accurate represented
related to #4042

```
In [1]: DataFrame([[1e-16,'foo',1e-8]],columns=list('ABC')).to_json()

ValueError: ujson currently cannot accurately format float data less
than 1e-15. A work-around is to multiply the data by
a large positive factor and divide on deseriliazation
```
