force_text incorrectly handles SafeBytes under PY3

[Thomas Achtemichuk]

Under python 3 & Django 1.8.18, 1.9.13, 1.10.7, 1.11 and master, calling force_text on an instance of SafeBytes causes a str to be returned rather than an instance of SafeText.

>>> from django.utils.safestring import SafeBytes, SafeText
>>> from django.utils.encoding import force_text
>>> type(force_text(SafeText('')))
django.utils.safestring.SafeText
>>> type(force_text(SafeBytes(b'')))
str

This causes byte strings run through mark_safe and rendered in a template to be incorrectly escaped.

>>> from django.template import Template, Context
>>> from django.utils.safestring import mark_safe
>>> Template('{{ x }}').render(Context({'x': mark_safe(b'&')}))
'&'
>>> Template('{{ x }}').render(Context({'x': mark_safe('&')}))
'&'