BUG: add between DateOffsets raises error
```python
>>> pd.offsets.Day(1) + pd.offsets.Day(1)
<2 * Days>
>>> pd.offsets.DateOffset(days=1) + pd.offsets.DateOffset(days=1)
TypeError                                 Traceback (most recent call last)
 in 
----> 1 pd.offsets.DateOffset(days=1) + pd.offsets.DateOffset(days=1)

pandas\_libs\tslibs\offsets.pyx in pandas._libs.tslibs.offsets.BaseOffset.__add__()

pandas\_libs\tslibs\offsets.pyx in pandas._libs.tslibs.offsets.apply_wraps.wrapper()

TypeError: Argument 'other' has incorrect type (expected datetime.datetime, got DateOffset)
```
I think they should give the same output.
```python
>>>pd.offsets.DateOffset(days=1) - pd.offsets.DateOffset(days=1)
<0 * DateOffsets: days=1>
```
And the substraction works fine.