Don't update deferred fields

[arnau126]

If update_fields is not specified, only update not-deferred fields.

Usage example:

people = People.objects.all().only('age', 'name')
for person in people:
    person.age += 1
    person.name = 'John'
People.objects.bulk_update(people)

which it is equivalent (but less efficient) to:

people = People.objects.all()
for person in people:
    person.age += 1
    person.name = 'John'
People.objects.bulk_update(people, update_field=['age', 'name'])

[aykut]

Many thanks, another great PR.