Slicing columns with mixed types <str>,<int> fails with ValueError
#### Code Sample, a copy-pastable example if possible

```python
In []:
df = pd.DataFrame({'test':1, 1:2, 2:3}, index=[0])
df.loc[:, 'test':]
Out[]:
   test  1  2
0     1  2  3

In []:
df.loc[:, 1:]
Out[]:
TypeError                                 Traceback (most recent call last)
<ipython-input-655-3b97c047dcab> in <module>()
      1 df = pd.DataFrame({'test':1, 1:2, 2:3}, index=[0])
----> 2 df.loc[:, 1:]
[snip]
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/indexes/base.py in _invalid_indexer(self, form, key)
   1574                         "indexers [{key}] of {kind}".format(
   1575                             form=form, klass=type(self), key=key,
-> 1576                             kind=type(key)))
   1577 
   1578     def get_duplicates(self):

TypeError: cannot do slice indexing on <class 'pandas.core.indexes.base.Index'> with these indexers [1] of <class 'int'>

In []:
df = pd.DataFrame({0:1, 1:2, 2:3}, index=[0])
df.loc[:, 1:]
Out[]:
   1  2
0  2  3
```
#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]
When having a columns with mixed types, e.g. `<str>, <int>` any slicing using the `int` columns causes a `ValueError`.

When you have just `int` columns slicing by `int` does not cause an issue.

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#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.2.2
pip: 10.0.1
setuptools: 39.0.1
Cython: 0.28.1
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.7.2
patsy: None
dateutil: 2.6.1
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: 1.3.0
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.6.0
</details>
