BUG: Resample on empty Period-Indexed Series returns Datetime-Indexed Series
#### Code Sample, a copy-pastable example if possible

`
In [7]: pd.Series([], pd.period_range(start='2000', periods=0)).resample('W').mean().index
Out[7]: DatetimeIndex([], dtype='datetime64[ns]', freq='W-SUN')
`
#### Expected Output

`
PeriodIndex([], dtype='int64', freq='W-SUN')
`

On pandas: 0.18.0
