df.to_csv() ignores encoding when given a file object or any other filelike object.
#### Code Sample, a copy-pastable example if possible
```python
import pandas as pd
import io

# !! NOTE
# This example uses `io.BytesIO`, however this also applies to file buffers that are
# returned by `io.open` (the `open` function) when opened in binary mode.
buf = io.BytesIO('a, b, 🐟\n1, 2, 3\n4, 5, 6'.encode('utf-8'))
df = pd.read_csv(buf)   # reads in fine using default encoding (utf-8)
buf = io.BytesIO()

df.to_csv(buf, encoding='utf-8')  # this should work, but doesn't.
# TypeError: a bytes-like object is required, not 'str'

buf = io.StringIO()
df.to_csv(buf, encoding='utf-8')  # this 'works', but should fail.  Data is passed in without encoding.
buf.getvalue()
# ',a, b, 🐟\n0,1,2,3\n1,4,5,6\n'
```
#### Problem description
Currently, the 'encoding' parameter is accepted *and* doesn't do anything when dealing with an in-memory object.  This is deceptive, and can introduce encoding flaws.

I *presume* that pandas just sets the encoding on the file it opens.  In the case of receiving an already-open filelike object, pandas should encode the string and attempt to write the bytes into the file.  If it  fails, that's a valid and appropriate failure, and that failure should be raised.

However, in the interest of backwards compatibility, if it fails, it should probably try to write the unencoded string into the file, and perhaps display a warning.

I'm on Pandas 0.23.4.
https://pandas-docs.github.io/pandas-docs-travis/

#### Expected Output
```python
# a buffer that contains the following:
b',a, b, \xf0\x9f\x90\x9f\n0,1,2,3\n1,4,5,6\n'
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.3-041903-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 4.0.0
pip: 9.0.1
setuptools: 39.0.1
Cython: None
numpy: 1.15.4
scipy: None
pyarrow: 0.11.1
xarray: None
IPython: 7.1.1
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
