Allow access to internal ResponseWriter in gzhttp handler

[jgimenez]

Sometimes users will want to use the http.Flusher, or http.Hijacker, etc. interfaces from an http.ResponseWriter, which gzhttp.ResponseWriter does not implement but the underlying one does.

It is common to provide an Unwrap function to access it, for example the standard library expects that (although I didn't find any official documentation on it). See http.ResponseController for example.

[klauspost]

Both http.Hijacker and http.Flusher are supported and the latter must be used for it to work.

Please add this to NoGzipResponseWriter and I will consider whether to include it. Be aware there is additional wrapping if the provided ResponseWriter supports hijacking.

[klauspost]

(also, you can just check the type and grab it directly - after all it is publicly available).

[jgimenez]

Makes sense, done.

My main motivation is for compatibility with http.ResponseController from the standard library, linked above, where I can't change how the introspection is done.

[klauspost]

LGTM