MultiIndex (and not flat Index) casts bool to int
#### Code Sample, a copy-pastable example if possible

```python
In [2]: mi = pd.MultiIndex.from_product([['a', 'b'], [0, 1]])

In [3]: mi.get_loc(('a', True))
Out[3]: 1

In [4]: mi.get_level_values(1).get_loc(True)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/nobackup/repo/pandas/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2513             try:
-> 2514                 return self._engine.get_loc(key)
   2515             except KeyError:

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5834)()

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5617)()

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine._get_loc_duplicates (pandas/_libs/index.c:6064)()

~/nobackup/repo/pandas/pandas/_libs/index_class_helper.pxi in pandas._libs.index.Int64Engine._maybe_get_bool_indexer (pandas/_libs/index.c:19249)()

KeyError: True

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-4-0c9f78315be2> in <module>()
----> 1 mi.get_level_values(1).get_loc(True)

~/nobackup/repo/pandas/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2514                 return self._engine.get_loc(key)
   2515             except KeyError:
-> 2516                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   2517 
   2518         indexer = self.get_indexer([key], method=method, tolerance=tolerance)

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5834)()

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5617)()

~/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine._get_loc_duplicates (pandas/_libs/index.c:6064)()

~/nobackup/repo/pandas/pandas/_libs/index_class_helper.pxi in pandas._libs.index.Int64Engine._maybe_get_bool_indexer (pandas/_libs/index.c:19249)()

KeyError: True

In [5]: mi.get_indexer([('a', True), ('b', False)])
Out[5]: array([1, 2])

In [6]: mi.get_level_values(1).get_indexer([True, False])
Out[6]: array([-1, -1])

```

#### Problem description

From https://github.com/pandas-dev/pandas/issues/18519#issue-277042995 , just to keep track of it (and add a test in #19074 , which fixes it).

#### Expected Output

``KeyError: True`` in ``Out[3]: 1`` too, ``Out[5]:`` like ``Out[6]:``.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.23.0.dev0+42.g93033151a
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1

</details>
