code: func Json(req *http.Request, userStruct FieldMapper) error {
  	err := jsonBinder(req, userStruct)
  	if len(err) > 0 {
  		return err
  	}
  
  	return nil
  }
docs: // Json deserializes a JSON request body into a struct you specify
  // using the standard encoding/json package (which uses reflection).
  // This function invokes data validation after deserialization.