Return headers from 404 fetch (except status/statusText)

Why is this change being made?

Previously when a request was made for a page that didn't exist, we were fetching the 404 page (404.html) but using the headers from the original request, which means the content was HTML but the headers were from the original response from GCS (which includes content-type: application/xml). This means that for a 404, we'd see a page like this:

It was HTML content but the content-type said application/xml.

This MR fixes that by using the headers from the 404.html fetch except for the status + status text.

This issue only affects the review app and not about.gitlab.com

Author Checklist

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