assignment with dataframe.loc fails (in some cases)
This is probably by design but I find this very unintuitive...

More context:

``` python
import pandas as pd
import numpy as np

data = pd.DataFrame(np.zeros(2), columns=['A'])
data.loc[:].A = 'HELLO'
print data

data = pd.DataFrame(np.zeros(2), columns=['A'])
data.A.loc[:] = 'HELLO'
print data

data = pd.DataFrame(np.zeros(2), columns=['A'])
data.loc[:1].A = 'HELLO'  # <-- FAILS
print data

data = pd.DataFrame(np.zeros(2), columns=['A'])
data.A.loc[:1] = 'HELLO'
print data
```

Output:

```
       A
0  HELLO
1  HELLO

       A
0  HELLO
1  HELLO

   A
0  0
1  0

       A
0  HELLO
1  HELLO
```
