API: New callable indexing makes storing functions in a Series difficult
Example of something that previously worked (before 7bbd031104ee161b2fb79ba6f5732910661f94f8):

``` .python
>>> s = pd.Series([lambda x: x] * 10)
>>> s[s.index > 5] = lambda x: x + 1
```

But now the second line tries to apply the function in the rhs to the elements of the Series, rather than assigning them (and throws an exception). This is very counter-intuitive when using `__setitem__` rather than calling `Series.where` directly.
