Make _attach_current_user a pure function

Mutating dictionaries is gross and doesn’t work as you’d expect. Better
to have the function return a new dictionary instead.

Means we can be explicit that `created_by` is one of the allowed params
when updating a service.

tests/app/notify_client/test_job_client.py

@@ -7,20 +7,19 @@ def test_client_creates_job_data_correctly(mocker, fake_uuid):
     template_id = fake_uuid
     original_file_name = 'test.csv'
     notification_count = 1
+    mocker.patch('app.notify_client.current_user', id='1')
 
     expected_data = {
         "id": job_id,
         "template": template_id,
         "original_file_name": original_file_name,
-        "notification_count": 1
+        "notification_count": 1,
+        "created_by": '1'
     }
 
     expected_url = '/service/{}/job'.format(service_id)
 
     client = JobApiClient()
-    mock_attach_user = mocker.patch(
-        'app.notify_client.job_api_client._attach_current_user',
-        return_value={'created_by': fake_uuid})
     mock_post = mocker.patch('app.notify_client.job_api_client.JobApiClient.post')
 
     client.create_job(job_id, service_id, template_id, original_file_name, notification_count)