Different Accept-Headers for prefetch (link rel="prefetch") and regular request

I believe this is lower level issue with the browser, but adding it here as well.
When a prefetch is sent, the `accept` header does not fully match what the browser would send.

E.g. in Chrome 99.0.4844.51

Prefetch request has:
```
accept: application/signed-exchange;v=b3;q=0.7,*/*;q=0.8
```

But visiting the page results in an accept header of:
```
accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
```

Since the accept header _may_ be used to determine the response type (e.g. here the notation of avif would result in a differing set of HTML) , the prefetch is effectively unused