Added a relationship deletion unit test

To delete a relationship, the corresponding value
is being overriden with None. Eg:
{
    'data': {
        'type': 'posts',
        'relationships': {
            'blog': {
                'data': None
            }
        }
    }
}

Author: Philipp Schärer

example/tests/test_views.py

@@ -145,6 +145,22 @@ def test_delete_to_one_relationship_should_fail(self):
         response = self.client.delete(url, data=json.dumps(request_data), content_type='application/vnd.api+json')
         assert response.status_code == 405, response.content.decode()
 
+    def test_delete_relationship_overriding_with_none(self):
+        url = '/entries/{}'.format(self.first_entry.id)
+        request_data = {
+            'data': {
+                'type': 'posts',
+                'relationships': {
+                    'blog': {
+                        'data': None
+                    }
+                }
+            }
+        }
+        response = self.client.patch(url, data=json.dumps(request_data), content_type='application/vnd.api+json')
+        assert response.status_code == 400, response.content.decode()
+        assert response.data[0]['detail'] == 'This field may not be null.'
+
     def test_delete_to_many_relationship_with_no_change(self):
         url = '/entries/{}/relationships/comment_set'.format(self.first_entry.id)
         request_data = {