Inconsistent dtype changes between multi-column assignment and single-column assignment
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame({'a': [0.0], 'b': 0.0})

df[['a', 'b']] = 0
print(df)
print()

df['b'] = 0
print(df)
```

Output:
```
     a    b
0  0.0  0.0

     a  b
0  0.0  0
```
#### Problem description

As you can see above, the column type doesn't change in multi-column assignment but changes from `float64` to `int64` in single-column assignment. This behavior seems quite inconsistent.

#### Expected Output
```
   a  b
0  0  0

   a  b
0  0  0
```
Both multi-column assignment and single-column assignment should result in the correct dtype.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 3b96ada3a17f5fcc8c32a238457075ec4dd8433a
python           : 3.6.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-52-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.utf8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0rc0+131.g3b96ada3a
numpy            : 1.16.3
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : 0.29.7
pytest           : 4.4.2
hypothesis       : 4.17.2
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
