.loc with (scalar, slice(None)) on MultiIndex does not drop first level
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame(index=pd.MultiIndex.from_product([[1,2], [3, 4], [5, 6]]), columns=['a', 'b'])

In [3]: df.loc[(1,), :]
Out[3]: 
       a    b
3 5  NaN  NaN
  6  NaN  NaN
4 5  NaN  NaN
  6  NaN  NaN

In [4]: df.loc[(1,3), :]
Out[4]: 
     a    b
5  NaN  NaN
6  NaN  NaN

In [5]: df.loc[(1,[3,4]), :]
Out[5]: 
         a    b
1 3 5  NaN  NaN
    6  NaN  NaN
  4 5  NaN  NaN
    6  NaN  NaN
```

#### Problem description

Pandas should drop a level when it is indexed with a scalar. This happens in the first two examples, but not in the third (level 1 is not dropped).

This is vaguely related to #10521 and closely related to #12827 (which should be reopened, by the way).

#### Expected Output

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

<details>

INSTALLED VERSIONS
------------------
commit: 6e56195fcb021e7c53301722804087ef2a83bc39
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: 0.22.0.dev0+275.g6e56195fc
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>
