shift() broken for datetime64 when used with fill_value
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
from pandas.testing import assert_series_equal

dt_series = pd.Series((pd.to_datetime('2020-01-01'), pd.to_datetime('2020-01-02')))

actual_result = dt_series.shift(1, fill_value=0)
expected_result = pd.Series((pd.to_datetime('1970-01-01'), pd.to_datetime('2020-01-01')))

assert_series_equal(actual_result, expected_result)
```
#### Problem description

Above code snippet works on version 0.25.3 but breaks with 1.0.1.

```
print(actual_result)
0                      0
1    1577836800000000000
dtype: object

print(expected_result)
0   1970-01-01
1   2020-01-01
dtype: datetime64[ns]
```

```
AssertionError: Attributes of Series are different
Attribute "dtype" are different
[left]:  object
[right]: datetime64[ns]
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-72-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200210
Cython           : None
pytest           : 5.3.2
hypothesis       : None
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.11.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
pytest           : 5.3.2
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
