[BUG] Adding a `DateOffset(day=...)` to a datetime column doesn't match Pandas

**Describe the bug**
See the reproducer

**Steps/Code to reproduce bug**
```python
In [1]: import pandas as pd

In [2]: from datetime import date

In [4]: pd.DataFrame({"a": [date(2020, 1, 1)]})["a"] + pd.DateOffset(day=1)
Out[4]:
0 2020-01-01 00:00:00
Name: a, dtype: object

```
```python

In [1]: %load_ext cudf.pandas

In [2]: import pandas as pd

In [3]: from datetime import date

In [5]: pd.DataFrame({"a": [date(2020, 1, 1)]})["a"] + pd.DateOffset(day=1)
/home/coder/cudf/python/cudf/cudf/pandas/fast_slow_proxy.py:28: PerformanceWarning: Non-vectorized DateOffset being applied to Series or DatetimeIndex.
return fn(*args, **kwargs)
Out[5]:
0 2020-01-01
Name: a, dtype: datetime64[s]
```

**Expected behavior**
Match pandas