Add support for Protobuf format response and unit test (#1479)

`Gin` now have the `protobufBinding` function to check the request format, but didn't have a protobuf response function like `c.YAML()`.
In our company [ByteDance](http://bytedance.com/), the largest internet company using golang in China, we use `gin` to transfer __Protobuf__  instead of __Json__, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.

render/protobuf.go

@@ -0,0 +1,33 @@
+// Copyright 2018 Gin Core Team.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package render
+
+import (
+	"net/http"
+
+	"github.com/golang/protobuf/proto"
+)
+
+type ProtoBuf struct {
+	Data proto.Message
+}
+
+var protobufContentType = []string{"application/x-protobuf"}
+
+func (r ProtoBuf) Render(w http.ResponseWriter) error {
+	r.WriteContentType(w)
+
+	bytes, err := proto.Marshal(r.Data)
+	if err != nil {
+		return err
+	}
+
+	w.Write(bytes)
+	return nil
+}
+
+func (r ProtoBuf) WriteContentType(w http.ResponseWriter) {
+	writeContentType(w, protobufContentType)
+}