Merge current v2.9 into v3.0

pkg/middlewares/customerrors/custom_errors.go

@@ -209,6 +209,15 @@ func (cc *codeCatcher) Flush() {
 	// Otherwise, cc.code is actually a 200 here.
 	cc.WriteHeader(cc.code)
 
+	// We don't care about the contents of the response,
+	// since we want to serve the ones from the error page,
+	// so we just don't flush.
+	// (e.g., To prevent superfluous WriteHeader on request with a
+	// `Transfert-Encoding: chunked` header).
+	if cc.caughtFilteredCode {
+		return
+	}
+
 	if flusher, ok := cc.responseWriter.(http.Flusher); ok {
 		flusher.Flush()
 	}