makeMissingDataFrame is broken
Did something change in iloc...? I could have sworn this worked and we talked about it during the PR. I was told _not_ to use values but to use iloc instead IIRC.

```
[~/]
[18]: pd.version.version
[18]: '0.13.1-254-g150f323'


from pandas.util.testing import _create_missing_idx
df = pd.util.testing.makeDataFrame()
density = .9
random_state = None

i, j = _create_missing_idx(*df.shape, density=density,
                           random_state=random_state)
df.iloc[i, j] = np.nan
```

This must be a copy because there's no assignment.

This still works

```
df.values[i, j] = np.nan
```
