to_csv and bytes on Python 3.
Is this desired behavior and something I need to work around or a bug? Notice the byte type marker is written to disk so you can't round-trip the data. This works fine in Python 2 with unicode AFAICT.

```
In [1]: pd.__version__
Out[1]: '0.15.2-252-g0d35dd4'

In [2]: pd.DataFrame.from_dict({'a': ['a', 'b', 'c']}).a.str.encode("utf-8").to_csv("tmp.csv")

In [3]: !cat tmp.csv
0,b'a'
1,b'b'
2,b'c'
```
