DatetimeIndex periods: Change in behavior between 0.17.1 and 0.18.0
This is not necessarily a bug, but a change in behavior that could not find reported elsewhere.

``` python
import pandas as pd,datetime as dt
pd.DatetimeIndex(freq=pd.tseries.offsets.DateOffset(months=6),end=dt.date(2016,1,1),
closed='right',periods=3)
```

In version 0.17.1 you get

``` python
DatetimeIndex(['2015-07-01', '2016-01-01'], dtype='datetime64[ns]', 
freq='<DateOffset: kwds={'months': 6}>')
```

however, in version 0.18.1 you get

``` python
DatetimeIndex(['2015-01-01', '2015-07-01', '2016-01-01'], dtype='datetime64[ns]', 
freq='<DateOffset: kwds={'months': 6}>')
```

The new behavior seems more intuitive. Is this change documented anywhere? Thanks!
