ERR: validate encoding on to_stata
It seems ``pandas`` in ``python3.5`` causes issues due to encoding. For example the following generates a corrupt output file

```python
import pandas as pd
df1 = pd.DataFrame(np.array([1,2,3,4]), columns=['var1'])
df1.to_stata('corrupt.dta', write_index=False, encoding='utf8')
```

while 

```python
df1.to_stata('not-corrupt.dta', write_index=False)
```

generates a correct file. I imagine this may be due to use of encoding and the difference in the treatment between ``python 2`` and ``python 3``, which breaks compatibility of scripts across ``python`` versions. I guess it would be nice if it does not take this option into account on ``python 3``, unless the error is caused by something else.
