Fixed #14877 -- repeated deletion using formsets

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 a primary key that was also a related field didn't convert
the pk value properly.