workerd should inject a User-Agent if none is specified in the request

We're failing the following subtests of `fetch/api/basic/request-headers.any.js` due to this:

- Fetch with POST with Blob body with mime type
- Fetch with PUT without body
- Fetch with POST with URLSearchParams body
- Fetch with POST with FormData body

Inserting a User-Agent is mentioned by s 4.5.15 of the fetch spec:

> If httpRequest’s [header list](https://fetch.spec.whatwg.org/#concept-request-header-list) [does not contain](https://fetch.spec.whatwg.org/#header-list-contains) `User-Agent`, then user agents should [append](https://fetch.spec.whatwg.org/#concept-header-list-append) (`User-Agent`, [default `User-Agent` value](https://fetch.spec.whatwg.org/#default-user-agent-value)) to httpRequest’s [header list](https://fetch.spec.whatwg.org/#concept-request-header-list).

-- https://fetch.spec.whatwg.org/#http-network-or-cache-fetch

They say _should_ though. Maybe WPT is wrong to fail us for not doing it, as it doesn't say _must_?