Pandas read_excel uses blank column heading as index
#### Problem description

When using Pandas read_excel function with an Excel file that has a column heading that is blank, Pandas will regard the column with the blank heading as the index.  

Consider this example file:  [https://tinyurl.com/y8br2js4](test_file.xlsx)
Within this file, column B does not have a column heading, it is just a blank cell.  The rest of the columns have names.

When running this code:
```python
import pandas as pd

xlsx_file = './Input/Test_File.xlsx'

test_df = pd.read_excel(xlsx_file,
                        index_col=None,
                        usecols='B, C, E, F')
```

The resulting dataframe will use column B as the index, even though index_col=None is specified.  If I add column A to usecols, the dataframe processes the file as expected.  

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.22.0
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: 0.8.0
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
