chore: encode: use FlushError instead of Flush (#6168)

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

modules/caddyhttp/encode/encode.go

@@ -202,15 +202,15 @@ func (enc *Encode) Match(rw *responseWriter) bool {
 	return enc.Matcher.Match(rw.statusCode, rw.Header())
 }
 
-// Flush implements http.Flusher. It delays the actual Flush of the underlying ResponseWriterWrapper
+// FlushError is an alternative Flush returning an error. It delays the actual Flush of the underlying
-// until headers were written.
+// ResponseWriterWrapper until headers were written.
-func (rw *responseWriter) Flush() {
+func (rw *responseWriter) FlushError() error {
 	if !rw.wroteHeader {
 		// flushing the underlying ResponseWriter will write header and status code,
 		// but we need to delay that until we can determine if we must encode and
 		// therefore add the Content-Encoding header; this happens in the first call
 		// to rw.Write (see bug in #4314)
-		return
+		return nil
 	}
 	// also flushes the encoder, if any
 	// see: https://github.com/jjiang-stripe/caddy-slow-gzip
@@ -221,7 +221,7 @@ func (rw *responseWriter) Flush() {
 		}
 	}
 	//nolint:bodyclose
-	http.NewResponseController(rw.ResponseWriter).Flush()
+	return http.NewResponseController(rw.ResponseWriter).Flush()
 }
 
 // Hijack implements http.Hijacker. It will flush status code if set. We don't track actual hijacked