requestbuilder.go
func (r *RequestBuilder) Option(key string, value interface{}) *RequestBuilder {
	var s string
	switch v := value.(type) {
	case bool:
		s = strconv.FormatBool(v)
	case string:
		s = v
	case []byte:
		s = string(v)
	default:
		// slow case.
		s = fmt.Sprint(value)
	}
	if r.opts == nil {
		r.opts = make(map[string]string, 1)
	}
	r.opts[key] = s
	return r
}
func (r *RequestBuilder) Option(key string, value interface{}) *RequestBuilder {
	var s string
	switch v := value.(type) {
	case bool:
		s = strconv.FormatBool(v)
	case string:
		s = v
	case []byte:
		s = string(v)
	default:
		// slow case.
		s = fmt.Sprint(value)
	}
	if r.opts == nil {
		r.opts = make(map[string]string, 1)
	}
	r.opts[key] = s
	return r
}
// Option sets the given option.