WSGI applications are required to return bytes.

[fgallaire]

Needs @jaraco advice about:
fgallaire/wsgiserver#2

[jaraco]

Graham makes a good argument. I want to remove as many references to ISO-8859-1 from the code as possible, and removing this leniency is one place where that can be done. The tests continue to pass if text is refused with a ValueError, so I'm inclined to simply reject text values. Thanks for raising this here.