HTTP 403 on Index Retrieval

When the index is retrieved on the first request, it fails with:
```
HTTP Error 403: Forbidden
Fetching RFC index failed. Connected to the internet? Behind proxy?
```
I am connected though, and can open `https://www.rfc-editor.org/in-notes/rfc-index.xml` in the browser.

I think one should be able to reproduce this by deleting the cache file so the the index has to be downloaded again.

Delete the cache file and retry the first index retrieval to confirm the 403. Inspect the index-download entry point and compare its request with the browser-accessible URL. Done means the RFC index downloads successfully and is cached without the HTTP 403.

Written by the indexing model from the issue text.