Extra empty row when saving to CSV with MultiIndex columns
If using MultiIndex as columns, saving to CSV adds an extra line with no data between the column headers and the data. I couldn't find information that this is something that should happen so it was unexpected to me.
#### Code Sample, a copy-pastable example if possible

In IPython:

``` python
import pandas as pd
pd.DataFrame({('a','b'): [1, 2], ('c','d'): [3, 4]}).to_csv('temp.csv', index=False)
cat temp.csv
```

Outputs:

```
a,c
b,d
,
1,3
2,4
```
#### Expected Output

I expected not to have an empty row between column names and the data:

```
a,c
b,d
1,3
2,4
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.2-gnu-1
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_DK.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 20.10.1
Cython: 0.24
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: 1.4
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: None
xlrd: 0.9.4
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.0
sqlalchemy: 1.0.12
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
```
