BUG: DateOffset replaces month with given value instead of adding it to the current month.
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
import pandas as pd
from pandas.tseries.offsets import DateOffset

print(pd.__version__)

df = pd.DataFrame(pd.date_range('2020-10-01', periods=3, freq='D'))
print(df[0] + DateOffset(month=1))

1.2.0.dev0+690.g3a043f2d2
0   2020-01-01
1   2020-01-02
2   2020-01-03
Name: 0, dtype: datetime64[ns]
```

#### Problem description

DateOffset replaces the month instead of adding to the current month
#### Expected Output
```python
           0
0 2020-11-01
1 2020-11-02
2 2020-11-03

