Don't call managers when resolving accessors

In Django-1.7 they have become callable, but calling them is usually not
what we want. (django 04a2a6b0)

Fixes https://github.com/bradleyayers/django-tables2/issues/211 in a way
similar to https://github.com/django/django/blob/87eedfff44d23a8600557bcd9fa3ebbea0f3841e/django/template/base.py#L793