When a server returns a 304 response with a strong validator, any other
stored fields must be updated if they are also present in the response.
This behaviour is described in RFC9111, sections 3.2 and 4.3.4.
The recent HTTP client refactor in 14e25ab9fe
caused feed refreshes to no longer make conditional requests. Prior to
the refactor, `client.WithCacheHeaders` handled this. Now this function
is split into `fetcher.WithETag` and `fetcher.WithLastModified` but
these functions are only declared and never actually used. Fix this by
calling them inside `handler.RefreshFeed`.
The recent HTTP client refactor in 14e25ab9fe
introduced a bug in which the global default User-Agent is no longer
used for requests. Unless a per-feed User-Agent exists, the Go standard
library's default User-Agent is used, which looks something like
"Go-http-client/1.1". To fix this, make RequestBuilder.WithUserAgent
take an additional argument, the default User-Agent, which will be used
if there is no per-feed User-Agent (i.e. it is an empty string).
Fixes#2188Fixes#2189
