DataFrame.to_csv is ignoring the `index` parameter
#### Code Sample, a copy-pastable example if possible

```python
In [16]: import pandas as pd

In [17]: pd.__version__
Out[17]: '0.22.0'

In [18]: df = pd.DataFrame([(1,2,3)],columns=("a","b","c"))

In [19]: df
Out[19]:
   a  b  c
0  1  2  3

In [20]: df.to_csv("/dev/stdout",mode="a",index=False)
a,b,c
1,2,3
```
#### Problem description

According to the docs, the `index` parameter of `to_csv` controls whether pandas will write out column names. The sample above shows that this parameter is not working correctly.

#### Expected Output
`1,2,3`
#### Output of ``pd.show_versions()``

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.5.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
