ValueError: cannot set a Timestamp with a non-timestamp
#### Code Sample, a copy-pastable example if possible

```
from io import StringIO
df = pd.read_csv(StringIO('''Date,A\n8/23/2017,a\n8/22/2017,b\n8/24/2017,c'''), parse_dates=['Date'])
print(df)
print(type(pd.datetime(2017, 8, 22).date()))
print(type(df['Date'][1]))
df_sub = df[df['Date'] == pd.datetime(2017, 8, 22).date()]

```
#### Error returned
```
Traceback (most recent call last):
  File "<pyshell#72>", line 1, in <module>
    df_sub = df[df['Date'] == pd.datetime(2017, 8, 22).date()]
  File "C:\Python35\lib\site-packages\pandas\core\ops.py", line 877, in wrapper
    res = na_op(values, other)
  File "C:\Python35\lib\site-packages\pandas\core\ops.py", line 806, in na_op
    y = libindex.convert_scalar(x, _values_from_object(y))
  File "pandas\_libs\index.pyx", line 494, in pandas._libs.index.convert_scalar
  File "pandas\_libs\index.pyx", line 509, in pandas._libs.index.convert_scalar
ValueError: cannot set a Timestamp with a non-timestamp
```

#### Problem description

I just upgraded to pandas version '0.21.0rc1'. The `df_sub` line above used to work in a prior version. Should a `Timestamp` and a `datetime.date` be comparable like it used to in a previous version? Not a huge issue for me but my intent is to just report it in-case it requires attention.

#### Expected Output
```
        Date  A
1 2017-08-22  b
```

#### Helpful Links
May be a similar or related issue: https://github.com/pandas-dev/pandas/issues/17965
`convert_scalar` location: https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/index.pyx#L494

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: English_United States.1252

pandas: 0.21.0rc1
pytest: None
pip: 9.0.1
setuptools: 20.10.1
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.0rc2
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.9999999
sqlalchemy: 1.1.11
pymysql: None
psycopg2: None
jinja2: 2.9.5
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
