Regression in exporting of multiindex column dataframes
#### Code Sample, a copy-pastable example if possible

```python
import pandas
pandas.DataFrame({(1, 2): 3, (1, 3): 1, (2, 1): 2, (2, 2): 0}, index=[0]).to_excel('test.xlsx')
```
#### Problem description

Expected behavior, observed with pandas v0.20.3

![image](https://user-images.githubusercontent.com/2069677/32562411-e0d4ad40-c4ae-11e7-8bc6-19b2bea4710b.png)

Behavior observed with pandas v0.21.0:

![image](https://user-images.githubusercontent.com/2069677/32562441-f7bdd70c-c4ae-11e7-9e93-b6ad26f8791a.png)

**Note**: I did not verify whether the issue is resolved on master
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-98-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_DK.UTF-8
LANG: en_US.UTF-8
LOCALE: en_DK.UTF-8

pandas: 0.21.0
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.6.0
Cython: 0.27.2
numpy: 1.12.1
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: 2.5.0b1
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>

For a reference, the working configuration:

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-98-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_DK.UTF-8
LANG: en_US.UTF-8
LOCALE: en_DK.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 36.2.2.post20170724
Cython: None
numpy: 1.12.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: 2.5.0a2
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
