Allow updates to the SMS prefix setting

This was already working, but adds a test to make sure that it works as
requested.

Still to do:
- migrate all services to having `True` or `False` in this column
- make column non-nullable
- start rejecting `None` as a possible value for this update

tests/app/service/test_rest.py

@@ -1592,6 +1592,39 @@ def test_prefixing_messages_based_on_sms_sender(
     assert service['prefix_sms_with_service_name'] == should_prefix
 
 
+@pytest.mark.parametrize('posted_value, stored_value, returned_value', [
+    (True, True, True),
+    (False, False, False),
+    (None, None, True),
+])
+def test_set_sms_prefixing_for_service(
+    client,
+    sample_service,
+    posted_value,
+    stored_value,
+    returned_value,
+):
+    data = {
+        'prefix_sms': posted_value,
+    }
+
+    auth_header = create_authorization_header()
+
+    resp = client.post(
+        '/service/{}'.format(sample_service.id),
+        data=json.dumps(data),
+        headers=[('Content-Type', 'application/json'), auth_header]
+    )
+    result = json.loads(resp.get_data(as_text=True))
+    assert resp.status_code == 200
+    assert result['data']['prefix_sms'] == stored_value
+    # This derived value will go away eventually, once we’ve done a migration
+    assert result['data']['prefix_sms_with_service_name'] == returned_value
+    # The derived value is dependent on the service sending from the platform’s
+    # default from number
+    assert result['data']['sms_sender'] == current_app.config['FROM_NUMBER']
+
+
 @pytest.mark.parametrize('today_only,stats', [
     ('False', {'requested': 2, 'delivered': 1, 'failed': 0}),
     ('True', {'requested': 1, 'delivered': 0, 'failed': 0})