Series.mask should accept same arguments as Series.where
`Series.mask` just performs a mask operation. `Series.where` accepts substitute values. (In the meantime, workaround is simple - just invert the where selection).

Version 0.15.1 docs:

```
Series.mask(cond)
     Returns copy whose values are replaced with nan if the inverted condition is True

Series.where(cond, other=nan, inplace=False, axis=None, level=None, try_cast=False, raise_on_error=True)
     Return an object of same shape as self and whose corresponding entries are from self where cond is True and otherwise are from other.

```
