HttpClient fs2 backend: Fail to read empty body when Content-Encoding is set

[ybasket]

The fs2 HttpClient (JDK11+) backend fails if a response has Content-Encoding: gzip set and a status code 204 (No Content) as the fs2 gunzip pipe expects the gzip header bytes even if the stream is entirely empty. A more graceful handling would help consuming an otherwise perfectly fine response.

The RFCs don't seem entirely clear whether this is legal, but it can happen when using http4s (see http4s/http4s#5056) and has been "fixed" in other HTTP clients before (see request/request#2177).