File mode in `to_csv` is ignored, when passing a file object instead of a path
#### Code Sample, a copy-pastable example if possible

```
>>> import pandas as pd
>>> df = pd.read_csv("example.csv")
>>> df.head()
   just  a  file
0     1  2     3
1     4  5     6
2     7  8     9
>>> with open("someother.csv", "wb") as f:
...     df.to_csv(f, mode="wb")
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python3.6/site-packages/pandas/core/frame.py", line 1524, in to_csv
    formatter.save()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1652, in save
    self._save()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1740, in _save
    self._save_header()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1708, in _save_header
    writer.writerow(encoded_labels)
TypeError: a bytes-like object is required, not 'str'
```
#### Problem description

When passing a file opened in binary mode to `df.to_csv` and also passing `mode='wb'`, this mode is ignored. I think it's because of these lines: https://github.com/pandas-dev/pandas/blob/master/pandas/io/common.py#L407-L411 and these ones: https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/format.py#L1660-L1662

It seems that `is_text` isn't passed, and so it assumes the default value of `True`

#### Expected Output
A file should just be written.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.20-1-lts
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.4.0
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.7.0
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 1.0b10
sqlalchemy: 1.2.3
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: 0.1.2
fastparquet: 0.1.3
pandas_gbq: None
pandas_datareader: None

</details>
