BUG: resample on PeriodIndex with 'W' causes recursion
```
In [32]: s = pd.Series(range(2), pd.period_range('1/1/1990', periods=2, freq='M'))

In [33]: s
Out[33]: 
1990-01    0
1990-02    1
Freq: M, dtype: int64

In [34]: s.resample('W').asfreq()
```
