requestbuilder.go
func (r *RequestBuilder) Header(name, value string) *RequestBuilder {
	if r.headers == nil {
		r.headers = make(map[string]string, 1)
	}
	r.headers[name] = value
	return r
}
func (r *RequestBuilder) Header(name, value string) *RequestBuilder {
	if r.headers == nil {
		r.headers = make(map[string]string, 1)
	}
	r.headers[name] = value
	return r
}
// Header sets the given header.