Possible bug in df.update
#### Code Sample, a copy-pastable example if possible

```python
test_json = [{"_id": 'a', 'date': datetime.now()}, {"_id": 'b', 'date': datetime.now()}]
test_df = pd.DataFrame(test_json)

new_df = test_df.copy()
new_df["date"] = None
new_df.update(test_df)

print(test_df.head())
print(new_df.head())

```
#### Problem description

When using update function with datetime data, it is automatically converted to timestamp, which for me it seems like an abnormal behaviour. Code from above would output

  _id                       date
0   a 2019-11-07 15:50:06.072158
1   b 2019-11-07 15:50:06.072158
  _id                 date
0   a  1573141806072158000
1   b  1573141806072158000

#### Expected Output

  _id                       date
0   a 2019-11-07 15:50:06.072158
1   b 2019-11-07 15:50:06.072158
  _id                 date
0   a 2019-11-07 15:50:06.072158
1   b 2019-11-07 15:50:06.072158

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.2
setuptools       : 41.0.1
Cython           : 0.29.13
pytest           : None
hypothesis       : None
sphinx           : 2.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
