infer_freq borks on PeriodIndex
Should PeriodIndex pass through infer_freq and return freqstr or should this be the user's problem? Either is a simple fix. The latter should just have a better error message. Right now

```
from pandas.tseries.api import infer_freq
from pandas import PeriodIndex

infer_freq(PeriodIndex(start="1/1/1990", periods=20, freq="M"))
```
