By replacing `XMLHttpRequest` with a `fetch` call, the helper function can be modernized to use async/await instead.
Note that the headers doesn't seem necessary to set now, since:
 - The Fetch API provides a method for accessing the response as *text*, which renders the "Content-type" header unnecessary.
 - According to https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name, the "Content-length" header isn't necessary.