Series index silently ignored when used as boolean mask to df.columns
#### Code Sample:

```python
import pandas as pd

test_data = pd.DataFrame({
    'a': [1, 2, 3],
    'b': [0, 0, 0],
    'c': [3, 6, 9]
}, index=['bababa', 'cacacaca', 'zazazazaza'])

bool_mask = pd.Series([False, True, True], index=['b','a','c'])

# note Series index order != DataFrame column order (by design)

print(test_data.loc[:, bool_mask])

#             a  c
#  bababa      1  3
#  cacacaca    2  6
#  zazazazaza  3  9

print(test_data.columns[bool_mask])

#  Index(['b', 'c'], dtype='object')

```
#### Problem description

Using an instance of pd.Series as a boolean mask to select columns in a dataframe works entirely as expected when using `df.loc[:, boolean_mask]` but when instead used to filter columns via `df.columns[bool_mask]` the index of the Series is silently ignored. 

If the index order of the boolean mask Series is identical to the column order of the dataframe, `df.columns[bool_mask]` works as expected but in the case where the orders differ,  `df.columns[bool_mask]` will return the wrong column names.


#### Expected Output

```python
            a  c
bababa      1  3
cacacaca    2  6
zazazazaza  3  9
Index(['a', 'c'], dtype='object')
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.13.3
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: 1.1.0
tables: 3.4.2
numexpr: 2.6.4
feather: 0.3.1
matplotlib: 2.0.2
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.0
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
