Change in behavior of DatetimeIndex + Offset
```python
import pandas as pd

offset = pd.tseries.offsets.Week(weekday=6)
idx = pd.DatetimeIndex(['1999-12-26', '2000-05-14']) 

assert (idx[0] + offset) == (idx + offset)[0]
```

bisected to https://github.com/pandas-dev/pandas/pull/18952

Before that the assertion was true, after it's false

```
# before
>>> idx + offset
DatetimeIndex(['2000-01-02', '2000-05-21'], dtype='datetime64[ns]', freq=None)

# after
>>> idx + offset
DatetimeIndex(['1999-12-26', '2000-05-14'], dtype='datetime64[ns]', freq=None)
```

cc @reidy-p is this an expected change due to #18952, or a new bug? If it's intentional, we should document that.