Thanks sebastien at clarisys.fr for the report and gautier
for the patch.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

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.

Thanks PaulM for the suggestion and Luke Granger-Brown and
Wiktor Kołodziej for the initial patch.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

Gives much nicer errors when it fails.

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.