.loc Multiindex DateTime slicing failures
related to #16637 and #14946

#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

dt_idx = pd.to_datetime(['2017-05-04','2017-05-05'])
m_idx = pd.MultiIndex.from_product([dt_idx,dt_idx], names=['Idx1','Idx2'])
df = pd.DataFrame(data=[[1,2],[3,4],[5,6],[7,6]], index=m_idx, columns=['C1','C2'])

level1 = df.index.get_level_values(1)
_mask = (level1 > '2017-05-04')

print(df.loc[(dt_idx[0]), 'C1'])              # WORKS OK
print(df.loc[(slice(None), _mask), 'C1'])     # WORKS OK
print(df.loc[('2017-05-04', _mask), 'C1'])    # WORKS OK
print(df.loc[(dt_idx[0], _mask), 'C1'])       # FAILS TypeError, invalid key

```
#### Problem description

Inconsistent behaviour of .loc slicer when combining with mask (or possibly other data types) leads to fail.
I believe this is either tangentially or directly related to other issues with similar headers but this may be a new variant. 

#### Expected Output

Failed line expected to be equivalent to the working line above.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None
LOCALE: en_GB.UTF-8
pandas: 0.20.2
pytest: 3.0.5
pip: 9.0.1
setuptools: 32.1.0.post20161217
Cython: 0.25.2
numpy: 1.11.2
scipy: 0.18.1
xarray: None
IPython: 5.1.0
sphinx: 1.5
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: None
matplotlib: 1.5.3
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.4
lxml: None
bs4: 4.5.1
html5lib: None
sqlalchemy: 1.1.4
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: 0.0.7
pandas_gbq: None
pandas_datareader: None


</details>
