dtype changed when DataFrame add DateOffset
#### Code Sample, a copy-pastable example if possible

```python
pd.DataFrame([pd.to_datetime('2018')]).dtypes
# 0    datetime64[ns]
# dtype: object
(pd.DataFrame([pd.to_datetime('2018')]) + pd.DateOffset(years=1)).dtypes
# 0    object
# dtype: object
```
#### Problem description
`Timestamp` + `DateOffset` = `Timestamp`, but `DataFrame(datetime64[ns])` + `DateOffset` = `DataFrame(object)`.

#### Expected Output
`DataFrame(datetime64[ns])` + `DateOffset` = `DataFrame(datetime64[ns])`. Don't change dtype.

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.23.0
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: 1.6.2
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.8.0
bs4: 4.5.3
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
