DataFrame.at indexing with date string promotes column type, while indexing with datetime casts value
#### Code Sample, a copy-pastable example if possible

```python
index = pd.date_range("2019-01-01", "2019-01-02", freq="D")
df = pd.DataFrame(index=index, columns=["A"]).fillna(1)

>>> df.at[pd.to_datetime('2019-01-01'), 'A'] = 0.98
>>> df
            A
2019-01-01  0  # cast to column dtype
2019-01-02  1
>>> df['A'].dtype
dtype('int64')

>>> df.at['2019-01-01', 'A'] = 0.98
>>> df
               A
2019-01-01  0.98  # column cast to value dtype
2019-01-02  1.00
>>> df['A'].dtype
dtype('float64')
```
#### Problem description
This issue arises when dealing with a Dataframe with a datetime index. When using `DataFrame.at[]` and indexing using a date string, the column dtype is promoted to the type of the provided value. However when using `DataFrame.at[]` and indexing using a datetime, the provided value is instead cast to the column's dtype. In the example, `0.98` is in this case indeed cast to 0.

#### Expected Output
That indexing `DataFrame.at[]` with either a date string or a datetime works the same.

#### Output of ``pd.show_versions()``
<details>

[paste the output of ``pd.show_versions()`` here below this line]
>>> pd.show_versions()
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.2.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.16.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.6.0
Cython           : None
pytest           : 4.3.0
hypothesis       : 4.7.11
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.7.7 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
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            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
