Web APIs must set a non-HTML content-type on all responses, including 300s, 400s and 500s

This recommendation comes from #18.

We should enforce this at the root level: if the result is a HTML-like content-type, we should force it to something else. This should be possible with a simple middleware.
Maybe that exists in a ready-to-use package already.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-982)

Start at the server's root-level middleware or request entry point and inspect how response content types are set. Verify the behavior for normal responses and 300-, 400-, and 500-level responses; done means no HTML-like content-type is returned by the Web APIs, including when errors occur.

Written by the indexing model from the issue text.