index locators cast bool to float (and not to int)
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.Index([0, 1, 2, 3]).get_loc(True) # right
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/home/pietro/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:

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

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5639)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/index_class_helper.pxi in pandas._libs.index.Int64Engine._check_type (pandas/_libs/index.c:18970)()

KeyError: True

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-2-31d89f73ae5f> in <module>()
----> 1 pd.Index([0, 1, 2, 3]).get_loc(True)

/home/pietro/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)

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

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5639)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/index_class_helper.pxi in pandas._libs.index.Int64Engine._check_type (pandas/_libs/index.c:18970)()

KeyError: True

In [3]: pd.Index([0, 1, 2, 3.]).get_loc(True) # wrong
Out[3]: 1
```

#### Problem description

Given that bools are not casted to ints (which I think is good), even less one would expect that they are casted to float.

#### Expected Output

``KeyError: True`` in both cases.

#### 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: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0.dev0+448.gdbec3c92e
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1+dev
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
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: 1.2.0
xlsxwriter: None
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None



</details>
