When a formset contained deletion for an existing instance, and the
instance was already deleted, django threw an exception. A common cause for
this was resubmit of the formset.
Original patch by Trac alias olau.
In addition this commit cleaned some code in _construct_form(). This
was needed as the primary key value the user submitted wasn't converted
correctly to python value in case the primary key field was also a
related field.
This also updates all dependent functionality, including modelform_factory
and modelformset_factory, and the generic views `ModelFormMixin`,
`CreateView` and `UpdateView` which gain a new `fields` attribute.
