Assignment using .ix and None incorrectly assigns NaN
When I try to initialize a new column with None, NaN is filled in instead.  For example:

```
import pandas as pd
df=pd.DataFrame(1, index=['A'], columns=['x'])
df.ix[:,"Grouping"]=None
df
```

results in 

```
   x  Grouping
A  1       NaN
```

Alternatively, if I assign a random number instead of the constant 1, I get None in the Grouping column

```
import pandas as pd
df=pd.DataFrame(np.random.randn(1,1), index=['A'], columns=['x'])
df.ix[:,"Grouping"]=None
df
```

```
          x Grouping
A  0.254445     None
```

It should result in None in both cases
