add audit/admin trace support for browser requests (#10947)

To support this functionality we had to fork
the gorilla/rpc package with relevant changes

pkg/rpc/encoder_selector.go

@@ -0,0 +1,48 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copyright 2020 MinIO, Inc. All rights reserved.
+// forked from https://github.com/gorilla/rpc/v2
+// modified to be used with MinIO under Apache
+// 2.0 license that can be found in the LICENSE file.
+
+package rpc
+
+import (
+	"io"
+	"net/http"
+)
+
+// Encoder interface contains the encoder for http response.
+// Eg. gzip, flate compressions.
+type Encoder interface {
+	Encode(w http.ResponseWriter) io.Writer
+}
+
+type encoder struct {
+}
+
+func (_ *encoder) Encode(w http.ResponseWriter) io.Writer {
+	return w
+}
+
+var DefaultEncoder = &encoder{}
+
+// EncoderSelector interface provides a way to select encoder using the http
+// request. Typically people can use this to check HEADER of the request and
+// figure out client capabilities.
+// Eg. "Accept-Encoding" tells about supported compressions.
+type EncoderSelector interface {
+	Select(r *http.Request) Encoder
+}
+
+type encoderSelector struct {
+}
+
+func (_ *encoderSelector) Select(_ *http.Request) Encoder {
+	return DefaultEncoder
+}
+
+var DefaultEncoderSelector = &encoderSelector{}