Fix admin app putting service into research mode

We changed the `update_service` method to only update indivdual
attributes of a service, and only allow it to update specified
attributes: 0cfe10639a

We neglected to specify `research_mode` as one of the allowed
attributes.

This broke the app’s ability to put a service in or out of research
mode.

This commit:
- makes sure the tests cover this eventuality
- fixes the bug by specifying `research_mode` as one of the allowed
  attributes

app/notify_client/service_api_client.py

@@ -89,6 +89,7 @@ class ServiceAPIClient(NotificationsAPIClient):
             'restricted',
             'email_from',
             'reply_to_email_address',
+            'research_mode',
             'sms_sender',
             'created_by',
             'branding',