Fix widgets.ImageInput for Django 1.11
Fix widgets.ImageInput for Django 1.11

The FileInput.format_value returns None instead of the passed value
in Django 1.11. Remove the usage of format_value since it was a noop
before Django 1.11 anyway

Fixes #2334

solarissmoke/django-oscar,sonofatailor/django-oscar,sonofatailor/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,sonofatailor/django-oscar,sasha0/django-oscar,solarissmoke/django-oscar,sasha0/django-oscar,django-oscar/django-oscar,django-oscar/django-oscar,sasha0/django-oscar,django-oscar/django-oscar,sonofatailor/django-oscar,sasha0/django-oscar,solarissmoke/django-oscar