BUG: unstacking with object columns defaults to None as a fill value
```
In [41]: df = DataFrame({'A' : [1,1,2], 'B' : list('aba'), 'C' : ['foo', np.nan, 'bar']}).set_index(['A','B'])

In [42]: df.unstack('B')
Out[42]: 
     C      
B    a     b
A           
1  foo   NaN
2  bar  None

In [43]: df.unstack('B', fill_value=np.nan)
Out[43]: 
     C     
B    a    b
A          
1  foo  NaN
2  bar  NaN
```

Introduced is this PR: https://github.com/pydata/pandas/pull/10246

you can make a case to do this as we DO support `None` as a fill value, but almost always even with object columns we use `np.nan`, unless very explicity set by the user.
