How argument ignored in to_timestamp
As discussed in https://groups.google.com/group/pydata/browse_thread/thread/180249bd46ed1944 the following fails:

``` python
def test_to_timstamp_how():
    months = pd.period_range('01-Aug-2012', periods=1, freq='M')
    assert months[0].asfreq('S','S').to_timestamp() == months.to_timestamp(how='start')[0]
#
```
