BUG/API: Consistency in .where() when setting with None for both inplace in a Series (GH7939)
closes #7939

@jreback not fully clear, but which of the two behaviours is it after this PR? The inplace (convert to NaN) or the non-inplace (keep None) ?
 I closed this because not really sure what (if anything) to do at this point. You can see the reference discussion on the issue. `None` is kind of a mess. But changing anything is extremely tricky as their really aren't enough tests to validate current or proposed behavior. and it is often ambiguous on what to do.

`np.nan` is the missing value indicator, so `None` should NOT be treated specially, but it IS in some places for convience. It is a can of worms. 
 ah, yes, I thought this was merged, my mistake. 
I know it is a mess, I just added some more of it :-) (https://github.com/pydata/pandas/issues/7939#issuecomment-51353012)
