fixed instantiation of UnicodeDecodeError in force_text

this seems to be a leftover from copying Django's force_text
function
(https://github.com/django/django/blob/master/django/utils/encoding.py#L98).
Note that Django uses its own DjangoUnicodeDecodeError that stores obj
on itself and then calls the super class without obj.

The bug doesn't really have an impact, since the `TypeError` thrown by using the wrong arguments is catched in ``to_unicode`` with a generic ``except Exception``.