.ix[int_number] falls back to positional indexing with MultiIndex with integers on first level
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.Series(range(10), index=range(10, 20)).ix[5]
[...]
KeyError: 5

In [3]: pd.Series(range(10), index=pd.MultiIndex.from_product([range(10, 20), [1]])).loc[5]
[...]
KeyError: 'the label [5] is not in the [index]'

In [4]: pd.Series(range(10), index=pd.MultiIndex.from_product([range(10, 20), [1]])).ix[5]
/usr/bin/ipython3:1: DeprecationWarning: 
.ix is deprecated. Please use
.loc for label based indexing or
.iloc for positional indexing

See the documentation here:
http://pandas.pydata.org/pandas-docs/stable/indexing.html#ix-indexer-is-deprecated
  #! /bin/sh
Out[4]: 5
```

#### Problem description

Yeah, I know, who cares about ``.ix`` anyway... it's just that this came out while refactoring, it is mistakenly tested, and I will push a fix, so this is to document the change in behaviour.

#### Expected Output

``KeyError`` also in ``Out[4]:``

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

<details>

INSTALLED VERSIONS
------------------
commit: f1ffc5fae06a7294dc831887b0d76177aec9b708
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+141.gf1ffc5fae
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>
