setitem(s) do not cast bool to float/int (while they cast int to float and vice-versa)
#### Code Sample, a copy-pastable example if possible

```python
In [2]: def cast_or_not(dtype1, dtype2):
   ...:     s = pd.Series(0, index=range(3), dtype=dtype1)
   ...:     s[0] = dtype2(1)
   ...:     return s.dtype == dtype1

In [3]: cast_or_not(bool, float)
Out[3]: True

In [4]: cast_or_not(float, bool)
Out[4]: False

In [5]: cast_or_not(bool, int)
Out[5]: True

In [6]: cast_or_not(int, bool)
Out[6]: False

In [7]: cast_or_not(float, int)
Out[7]: True

In [8]: cast_or_not(int, float)
Out[8]: True
```

#### Problem description

It's not necessarily obvious that booleans should be considered as a subset of integers (and floats), but given that we do cast floats to booleans, and floats and integers both ways, one would expect that booleans could also be casted to floats (and while it is true that ``bool(3)`` and ``bool(1)`` are the same thing, so the mapping is not entirely trivial, the same could be said for ``int(1.5)`` and ``int(1.0)``).

(This is not specific of ``__getitem__``, and also applies to ``.loc`` and ``.iloc``)

#### Expected Output

``True`` in all cases above.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+116.g9e982e186
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.25.2
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1153+gff6786446
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>
