datetime64 is converted to int64 when assigning to a new field
#### Code Sample, a copy-pastable example if possible

```python
# First, create a `Series` with `dtype` int64 or float64 or object (any type but datetime64).
series = pd.Series({'value': 123})
# >>> series
# value    123
# dtype: int64

# Setting a new datetime64 value, and it will be converted to int64.
series.at['date'] = datetime.datetime.now()
# >>> series
# value                    123
# date     1520605566114680000
# dtype: object

# If assigning to a existed field, it won't be auto-converted.
series.at['date'] = datetime.datetime.now()
# >>> series
# value                           123
# date     2018-03-09 14:26:19.945935
# dtype: object
```
#### Problem description

When assigning a `datetime64` value to a new field of a non-`datetime64` `Series`, it will be converted to `int64`.
While assigning to a existed field, it won't be converted.

#### Expected Output
A `datetime64` value should never be implicitly converted to `int64`.

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

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 38.5.2
Cython: None
numpy: 1.14.1
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.0
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.2
lxml: None
bs4: 4.6.0
html5lib: None
sqlalchemy: None
pymysql: 0.7.11.None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
