Pass client_id through to user.json

Signonotron2 requires you to pass the client_id through to the user.json
endpoint so that you can't use any apps token to gain access.

https://github.com/alphagov/signonotron2/blob/a2b3326de954300911d984aa1d191c71700846e9/test/integration/api_authentication_test.rb#L18
Merge pull request #370 from alphagov/feature/signon-client-id

Pass client_id through to user.json