Relationship deletion

Allow the deletion of relationships using the `relationships` values of a request, eg:

{
  "data": {
     "id": 1,
     "type": "model",
     "attributes": {
       "name": "test-model",
       "number": 1
     },
     "relationships": {
       "other-model": {"data": null}
     }
  }
}

See http://jsonapi.org/format/#crud-updating-to-one-relationships

Author: Philipp Schärer

rest_framework_json_api/parsers.py

@@ -40,7 +40,7 @@ def parse_relationships(data):
         parsed_relationships = dict()
         for field_name, field_data in relationships.items():
             field_data = field_data.get('data')
-            if isinstance(field_data, dict):
+            if isinstance(field_data, dict) or field_data is None:
                 parsed_relationships[field_name] = field_data
             elif isinstance(field_data, list):
                 parsed_relationships[field_name] = list(relation for relation in field_data)