infer_freq fails on Index
Continuing with the my projects are the pandas test suite theme of issue reports. This broke for me on a recent upgrade. 

Ok.

```
pd.infer_freq(['2004-01', '2004-02', '2004-03', '2004-04'])
```

No longer works.

```
pd.infer_freq(pd.Index(['2004-01', '2004-02', '2004-03', '2004-04']))
```

Seems like this should still work.

```
[12]: pd.version.version
[12]: '0.13.1-254-g150f323'
```
