Using the Headers class from the Fetch API.
Before, the added test would fail: `_requestInternal()`, not finding a header
named `Content-Type` (case sensitive), would set it to
`application/x-www-form-urlencoded`. XMLHttpRequest, upon being given both
`content-type`: `application/json`) and `Content-Type`:
`application/x-www-form-urlencoded`, would helpfully merge the two, producing
`content-type`: `application/json, application/x-www-form-urlencoded`. That's
obviously not the correct behavior.
f6dd47dd1f