BUG: DataFrame.update changes type of updated values
- [x] I have checked that this issue has not already been reported.

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

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



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**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import pandas as pd
df1 = pd.DataFrame.from_dict({"A": [1, 2, 3]})
df2 = pd.DataFrame.from_dict({"A": [4,  5]})
print(df1["A"])
df1.update(df2)
print(df1["A"])
```

Output:

```
# df1 before the update
0    1
1    2
2    3
Name: A, dtype: int64

# df1 after the update
0    4.0
1    5.0
2    3.0
Name: A, dtype: float64
```

#### Problem description
Update of int columns changes their type to float, that is unexpected behavior. And this happens only if the other dataframe does not have all indices of this dataframe. I would expect the type to be conserved...

#### Expected Output

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-957.el7.x86_64
Version          : #1 SMP Thu Oct 4 20:48:51 UTC 2018
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.19.5
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.6.0.post20210108
Cython           : 0.29.15
pytest           : 5.3.5
hypothesis       : None
sphinx           : 3.4.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.2
fsspec           : 2021.06.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : 0.16.2
xlrd             : None
xlwt             : None
numba            : None
1
​

</details>
