Handle HEAD responses with Brotli decoder

[Kludex]

Originally opened by @sethmlarson on 2019-08-19 02:23:06 in encode/httpx

Currently if you receive a response with Content-Encoding: br set and no body we get an error because Brotli doesn't like being called on an empty stream.

[Kludex]

Originally posted by @tomchristie on 2019-08-19T10:09:07Z

More generally: we should make sure not to call any decoder if the body is empty.

[Kludex]

Originally posted by @tomchristie on 2019-08-19T11:10:46Z

On reviewing #237 I think we should just call into the decoders, but that we should just make sure that all the decoder implementations correctly handle b"" input, and all correctly return b"" outputs in that case.

Ideally a test case for this issue would be parameterized to ensure correct behavior here from all our decoder classes.