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
