Fix a bug in the _create_formsets helper function

This commit fixes a bug which was introduced in Django 1.7.
The _create_formsets is a helper function that is used inside the changeform_view
method in several places. To generate formsets, it first checks if we are inside an add view
by checking if obj.pk resolves to True, that is, if obj was recently created and hasn't had
a primary key value assigned yet. Unfortunately, this is a heuristic and isn't always appropriate.
In particular, this method doesn't work if primary key is explicitly being set in the creation form.
The proposed result is to pass another parameter to _create_formsets.