Mixing strings and tuples in an index sometimes throws in an exception when selecting columns.
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({"a":[1], ("b", "c"):[2]})
df[["a", ("b", "c")]]
```
results in 
```ValueError: setting an array element with a sequence```

#### Problem description

Arguably this is a bug with np.asarray() (https://github.com/numpy/numpy/issues/12359) but it may be possible to work around it in pandas, e.g. asarray_tuplesafe() seems to be intended to handle this, but in this case does not.

#### Expected Output

```
   a  (b, c)
0  1       2
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.17.0-3rodete2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: None
pip: None
setuptools: None
Cython: None
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 2.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2018.7
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.5
feather: None
matplotlib: None
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
