Off By One for PeriodIndex.asfreq?
I haven't looked into this at all but is this the correct behavior?

```python
In [1]: import pandas as pd;

In [2]: p1 = pd.PeriodIndex(['2011-01-01', '2011-01-02', '2011-01-03'], freq='2D')

In [3]: p1
Out[3]: PeriodIndex(['2011-01-01', '2011-01-02', '2011-01-03'], dtype='period[2D]', freq='2D')

In [4]: p1.asfreq('D')
Out[4]: PeriodIndex(['2011-01-02', '2011-01-03', '2011-01-04'], dtype='period[D]', freq='D')
```

cc @jbrockmendel since I think you've been active here recently.