Error when indexing with `.loc` on index with mixed types (integer and string)
#### Code Sample

```python
import pandas as pd

df = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], 
                               index=[1, "a", 0], 
                               columns=["a", "b", "c"])

print(df.loc[[1, 0], ["a", "b"]])
```

Returns

```
   a  b
a  4  5
1  1  2
```

Currently on version `0.22.0`

#### Problem description

I tried to hunt down an issue that had this same problem because I doubt that I'm the first to report this, but I didn't see one (in open or closed lists) -- I certainly could have missed it and, if so, I apologize.

The problem is that when the index has a mixed type with integers (and something else) that it treats the selection of `.loc` as if it were `.iloc`.

In my case, I expected to get:

```
   a  b
1  1  2
0  7  8
```

but instead it pulled out the 1st and 0th rows.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.16.6-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.5.0

</details>
