Series/Dataframe to_csv inconsistency when path=None [0.14.1]

It looks like `Series.to_csv` and `DataFrame.to_csv` differ in their handling of `path=None`:

``` python
In [1]: import pandas as pd

In [2]: print pd.Series([1, 1, 1], index=[1, 2, 3]).to_csv(None)
None

In [3]: print pd.DataFrame([1, 1, 1], index=[1, 2, 3]).to_csv(None)
,0
1,1
2,1
3,1

```

I haven't gone digging yet but I expect these two methods share enough code that this is a trivial fix.
