to_csv with UTF16 Incorrectly Treats BOM as column
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame([['foo']])
In [4]: df.to_csv('afile.csv', encoding='utf16')
In [5]: with open('afile.csv', 'rb') as afile:
   ...:     print(afile.read())
   ...:
b'\xff\xfe,\x000\x00\n\x000\x00,\x00f\x00o\x00o\x00\n\x00'

```
Note the comma after the BOM `'\xff\xfe` - this has the unintended side effect of creating a malformed structure especially when trying to read back in

<details>

INSTALLED VERSIONS
------------------
commit: 9d5f1105c7335b790dc58914bc04b009243dc975
python: 3.7.2.final.0
python-bits: 64
OS: Darwin
OS-release: 18.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.25.0.dev0+587.g9d5f1105c
pytest: 4.3.0
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.7
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.3.0
sphinx: 1.8.4
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.0
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.5
lxml.etree: 4.3.1
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.2.18
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: 0.2.0
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>