For some reason, some misconfigured servers return an empty document when
queried over webfinger. Since an empty document does not lead to a parse
error, the error is not caught properly and triggers uncaught exceptions
This PR fixes that by immediately erroring out with `Webfinger::Error` on
getting an empty response.
There are edge cases where requests to certain hosts timeout when
using the vanilla HTTP.rb gem, which the goldfinger gem uses. Now
that we no longer need to support OStatus servers, webfinger logic
is so simple that there is no point encapsulating it in a gem, so
we can just use our own Request class. With that, we benefit from
more robust timeout code and IPv4/IPv6 resolution.