Overflow Error writing pandas 0.21.0 to_json in windows 7
Hi - I recently upgraded to pandas version 0.21.0 from version 0.20.3 and got the following error writing a dataframe `to_json`:
`OverflowError: Unsupported UTF-8 sequence length when encoding string`.

The traceback looks like this:
![image](https://user-images.githubusercontent.com/16763201/33522468-2cad7276-d7b3-11e7-9e50-5b2778ab1892.png)

As you can see it fails on converting dates(?).  My dataframe has no dates.
Here is how my df looks:
![image](https://user-images.githubusercontent.com/16763201/33522655-3d33c70e-d7b7-11e7-8e50-d0cd10463187.png)

However `to_json` does work for these cases:
`df = pd.DataFrame(np.random.randint(0,100,size=(5, 4)), columns=list('ABCD'))` 
and also for a standard `string`
`df['some_string'] = 'blah'`

- The same exact dataframe **does not** throw an error in 0.20.3. 






