DOC/BUG: error with .at indexer with datimeindex and enlargement
From docs: http://pandas-docs.github.io/pandas-docs-travis/indexing.html#fast-scalar-value-getting-and-setting

```
dates = date_range('1/1/2000', periods=8)
df = DataFrame(randn(8, 4), index=dates, columns=['A', 'B', 'C', 'D'])
df.at[dates[-1]+1, 0] = 7
```

does now raise an error
