allow show_url for all file fields
Merge branch 'master' of github.com:kakulukia/django-rest-framework

# By kakulukia
# Via kakulukia
* 'master' of github.com:kakulukia/django-rest-framework:
  allow to work like in django admin which also tries to call the given methon on the object itself
  Update fields.py

Conflicts:
	rest_framework/fields.py