base.Model __str__ sometimes returns unicode on Python 2

[Kevin Turner]

base.py contains from __future__ import unicode_literals

so the thing that Model.__str__ returns is a unicode object in the case where hasattr(self, '__unicode__') is False.

There's a force_text().encode() on the other case; probably both cases should use smart_str.

Discovered under Django 1.7 but this looks to still be the case in the master branch.