Fixed the many to many behavior.
Fixed the reversed relation for virtual fields (generic foreign keys).
Merge remote-tracking branch 'reference/master' into bugfix/nested_through_many_to_many
Merge remote-tracking branch 'reference/master' into bugfix/nested_through_many_to_many

Conflicts:
	rest_framework/serializers.py
Removed the former virtual field list.
Deal with reversed GenericFK.
Added an test on updates through many to many field.
Failing test case: an item that wasn't in the relation before is created although it already exist.
Merge remote-tracking branch 'origin/master' into bugfix/nested_through_many_to_many
Try to recover an object by its ID before duplicating it.
Merge pull request #1 from linovia/bugfix/nested_through_many_to_many

Bugfix/nested through many to many