Simplify code (#2004)

- Use buf.String instead of converison
- Remove redundant return

render/render_test.go

@@ -45,7 +45,7 @@ func TestRenderMsgPack(t *testing.T) {
 	err = codec.NewEncoder(buf, h).Encode(data)
 
 	assert.NoError(t, err)
-	assert.Equal(t, w.Body.String(), string(buf.Bytes()))
+	assert.Equal(t, w.Body.String(), buf.String())
 	assert.Equal(t, "application/msgpack; charset=utf-8", w.Header().Get("Content-Type"))
 }