code: func JSONKeyMapper(key string, sourceTag, destTag reflect.StructTag) (string, string) {
  	if s := destTag.Get("json"); len(s) > 0 {
  		return strings.SplitN(s, ",", 2)[0], key
  	}
  	if s := sourceTag.Get("json"); len(s) > 0 {
  		return key, strings.SplitN(s, ",", 2)[0]
  	}
  	return key, key
  }
docs: // JSONKeyMapper uses the struct tags on a conversion to determine the key value for
  // the other side. Use when mapping from a map[string]* to a struct or vice versa.