Python 3 writing to_csv file ignores encoding argument.
``` python
# is missing the UTF8 BOM (encoded with default encoding UTF8)
with open('path_to_f', 'w') as f:
    df.to_csv(f, encoding='utf-8-sig')

# is not missing the UTF8 BOM (encoded with passed encoding utf-8-sig)
df.to_csv('path_to_f', encoding='utf-8-sig')
```

I expect:

``` python
with open('path_to_f', 'w') as f:
    df.to_csv(f, encoding='utf-8-sig')
```

To crash with `TypeError: write() argument must be str, not bytes`

and I expect:

``` python
with open('path_to_f', 'wb') as f:
    df.to_csv(f, encoding='utf-8-sig')
```

To write the file correctly.
### Copy pasta

``` python
#!/usr/bin/env python3
import pandas as pd
df = pd.DataFrame()
with open('file_one', 'w') as f:
    df.to_csv(f, encoding='utf-8-sig')

assert open('file_one', 'rb').read() == b'""\n'

# is not missing the UTF8 BOM (encoded with passed encoding utf-8-sig)
df.to_csv('file_two', encoding='utf-8-sig')
assert open('file_two', 'rb').read() == b'\xef\xbb\xbf""\n'
```
