Fix django-json-api#784 Clear m2m relationships instead of deleting them during PATCH

Calling PATCH on an M2M RelationshipView when there were already some relationships was breaking. This was because the related objects were being deleted instead of just clearing the relationship and starting afresh.

Author: rohithpr

rest_framework_json_api/views.py

@@ -286,6 +286,8 @@ def remove_relationships(self, instance_manager, field):
             for obj in instance_manager.all():
                 setattr(obj, field_object.name, None)
                 obj.save()
+        elif hasattr(instance_manager, 'clear'):
+            instance_manager.clear()
         else:
             instance_manager.all().delete()