infer_freq on a series
Somewhat related to #6273 . I'm fairly certain though not positive that this also used to work on 0.12.x.

```
df = pd.DataFrame(np.random.rand(5,3))
df['date'] = ['1-1-1990', '2-1-1990', '3-1-1990', '4-1-1990', '5-1-1990']
pd.infer_freq(df['date'])
```

Raises Series has no attribute `is_monotonic`. I guess `infer_freq` now expects either an Index or ....?

```
[22]: pd.version.version
[22]: '0.13.0-472-g9109812'
```
