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/render_test.go

@@ -15,6 +15,8 @@ import (
 	"strings"
 	"testing"
 
+	testdata "github.com/gin-gonic/gin/testdata/protoexample"
+	"github.com/golang/protobuf/proto"
 	"github.com/stretchr/testify/assert"
 	"github.com/ugorji/go/codec"
 )
@@ -265,6 +267,35 @@ func TestRenderYAMLFail(t *testing.T) {
 	assert.Error(t, err)
 }
 
+// test Protobuf rendering
+func TestRenderProtoBuf(t *testing.T) {
+	w := httptest.NewRecorder()
+	reps := []int64{int64(1), int64(2)}
+	label := "test"
+	data := &testdata.Test{
+		Label: &label,
+		Reps:  reps,
+	}
+
+	(ProtoBuf{data}).WriteContentType(w)
+	protoData, err := proto.Marshal(data)
+	assert.NoError(t, err)
+	assert.Equal(t, "application/x-protobuf", w.Header().Get("Content-Type"))
+
+	err = (ProtoBuf{data}).Render(w)
+
+	assert.NoError(t, err)
+	assert.Equal(t, string(protoData[:]), w.Body.String())
+	assert.Equal(t, "application/x-protobuf", w.Header().Get("Content-Type"))
+}
+
+func TestRenderProtoBufFail(t *testing.T) {
+	w := httptest.NewRecorder()
+	data := &testdata.Test{}
+	err := (ProtoBuf{data}).Render(w)
+	assert.Error(t, err)
+}
+
 func TestRenderXML(t *testing.T) {
 	w := httptest.NewRecorder()
 	data := xmlmap{