Fast-indexing .at not working in 0.21.(.1)
#### Code 
```python

print df.at[date, 'column name']

```
#### Problem description

Since upgrading pandas to 0.21.0 and then 0.21.1, my code using fast indexing `.at` isn't working anymore. I get this error:

```
TypeError: only integer scalar arrays can be converted to a scalar index
```

`date` is a `<class 'pandas._libs.tslib.Timestamp'>` and is in the index of the DataFrame (simple DataFrame without multi-index). 

Doing `type(df.index[0])` also gives me `<class 'pandas._libs.tslib.Timestamp'>`

Any idea? Nothing changed in the docs on `.at`

Thanks

### Edit

I just did a simple test:

```
tmp = pd.dataframe(1, index=pd.date_range('2017-01-01','2017-02-02'), columns=['col1','col2'])
date = tmp.index[0]
tmp.at[date, 'col1']
```

and this is working... Very puzzling.
