Inappropriate/inconsistent behavior in data assignment (v. 0.13)
I find it convenient to store lists or series as elements in a dataframe. However, it seems panda gets confused if the iterable is the same length as the number of rows in the dataframe. Here's a minimal example:

``` python
matchmat_test = pd.DataFrame(index=[0,1], 
                             columns=[1], dtype=list)
matchmat_test.ix[0,0] = [1,2,3]
```

Returns:

```
           0
0  [1, 2, 3]
1        NaN
```

If we then assign to the same element again:

``` python
matchmat_test.ix[0,0] = [1,2]
```

we get:

```
 0
0  1
1  2
```
