* Make it an error to supply empty API credentials, like this:
`https://danbooru.donmai.us/posts.json?login=&api_key=`. Some clients
did this for some reason.
* Make it so that the `login` and `api_key` params are only allowed as
URL params, not as POST or PUT body params. Allowing them as body
params could interfere with the `PUT /api_keys/:id` endpoint, which
takes an `api_key` param.