diff of datetime column with NaT does not result into TimeDelta 
#### Code Sample, a copy-pastable example if possible

```python
In[40]: df = pd.DataFrame([[0]], dtype="datetime64[ns]")

In[46]: df[0] - pd.to_datetime("nat")
Out[47]: 
0   NaT
Name: 0, dtype: datetime64[ns]

In[47]: df[0] - pd.to_datetime("2017")
Out[48]: 
0   -17167 days
Name: 0, dtype: timedelta64[ns]

```
#### Problem description

Operation - on datetimes should always return a timedelta.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-35-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 36.2.7
Cython: None
numpy: 1.13.3
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.14
pymysql: None
psycopg2: 2.7.3 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
