DOC: Series.sum skipna= param defaults to None
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.sum.html

Docs for pandas.Series.sum state that "skipna=None" in the function name, but then reads "skipna : boolean, default True" in the "parameters" section.

**It is not clear whether the default is "None" or "True".** 

Many thanks.
