Problem in comparisons for DataFrame with pd.NaT
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import datetime as dt

tmp = pd.DataFrame.from_records(data=[[dt.datetime(1999,12,31), pd.NaT], [pd.NaT, dt.datetime(1999,12,31)]])
print(tmp.iloc[0]==dt.datetime(1999,12,31))
print(tmp==dt.datetime(1999,12,31))
```

Output
(First print statement Series comparison)
0     True
1    False

(Second print statement DataFrame)
      0     1
0  True  True
1  True  True

#### Problem description

When comparing a DataFrame containing pd.NaT (among other datetimes) with a single datetime results appears as True (second print statement). 
This is different from the behaviour for Series (first print statement), which corresponds to what one would expect.

#### Expected Output

Second print statement should be
True False
False True 

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-30-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.15.0
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
