Different behaviour when assigning None using loc[] and []
```
import pandas as pd
import numpy as np 

ser = pd.Series(['a', 'b', np.nan])
print ser

ser.loc[ser.isnull()] = None
print ser

ser[ser.isnull()] = None
print ser 
```

Output

```
0      a
1      b
2    NaN
dtype: object
0       a
1       b
2    None
dtype: object
0      a
1      b
2    NaN
dtype: object
```

I am not sure whether this behaviour is intentional. Using .loc[], None was correctly assigned but when using [], NaN was assigned instead. Thanks

I am using version 0.14.0-169-g7684b6e
