fix: double compression.

vendor/github.com/NYTimes/gziphandler/wrapper.go

@@ -0,0 +1,58 @@
+package gziphandler
+
+import (
+	"bufio"
+	"net"
+	"net/http"
+)
+
+const (
+	contentEncodingHeader = "Content-Encoding"
+)
+
+// ----------
+
+// http.ResponseWriter
+// http.Hijacker
+type GzipWriter interface {
+	Header() http.Header
+	Write([]byte) (int, error)
+	WriteHeader(int)
+	Hijack() (net.Conn, *bufio.ReadWriter, error)
+	Close() error
+	SetResponseWriter(http.ResponseWriter)
+	setIndex(int)
+	setMinSize(int)
+}
+
+func (w *GzipResponseWriter) SetResponseWriter(rw http.ResponseWriter) {
+	w.ResponseWriter = rw
+}
+
+func (w *GzipResponseWriter) setIndex(index int) {
+	w.index = index
+}
+
+func (w *GzipResponseWriter) setMinSize(minSize int) {
+	w.minSize = minSize
+}
+
+// --------
+
+type GzipResponseWriterWrapper struct {
+	GzipResponseWriter
+}
+
+func (g *GzipResponseWriterWrapper) Write(b []byte) (int, error) {
+	if g.gw == nil && isEncoded(g.Header()) {
+		return g.ResponseWriter.Write(b)
+	}
+	return g.GzipResponseWriter.Write(b)
+}
+
+func isEncoded(headers http.Header) bool {
+	header := headers.Get(contentEncodingHeader)
+	// According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding,
+	// content is not encoded if the header 'Content-Encoding' is empty or equals to 'identity'.
+	return header != "" && header != "identity"
+}