I have recently stumbled upon this bug, and I can present the example and a solution I've used.
The issue happens when we try to parse x-www-form-urlencoded of type bytes:
```
>>> from urllib.parse import urlencode, parse_qs
>>> urlencode([('v', 'ö')])
'v=%C3%B6'
>>> parse_qs('v=%C3%B6')
{'v': ['ö']}
>>> parse_qs(b'v=%C3%B6')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.6/urllib/parse.py", line 669, in parse_qs
    encoding=encoding, errors=errors)
  File "/usr/lib64/python3.6/urllib/parse.py", line 722, in parse_qsl
    value = _coerce_result(value)
  File "/usr/lib64/python3.6/urllib/parse.py", line 103, in _encode_result
    return obj.encode(encoding, errors)
UnicodeEncodeError: 'ascii' codec can't encode character '\xf6' in position 0: ordinal not in range(128)
```
This happens in the parse_qsl function because _coerce_result is a synonym of _encode_result and is called with default parameter encoding='ascii'. As far as I understand, it should be called with the encoding parameter of the parse_qsl function:
```
742c742
<             name = _coerce_result(name)
---
>             name = _coerce_result(name, encoding=encoding, errors=errors)
745c745
<             value = _coerce_result(value)
---
>             value = _coerce_result(value, encoding=encoding, errors=errors)
```
I am not sure whether I should commit this to the repo and create a pull request, as described in the devguide.